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SELECT(7)		 PostgreSQL 10.1 Documentation		     SELECT(7)

NAME
       SELECT, TABLE, WITH - retrieve rows from a table or view

SYNOPSIS
       [ WITH [ RECURSIVE ] with_query [, ...] ]
       SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
	   [ * | expression [ [ AS ] output_name ] [, ...] ]
	   [ FROM from_item [, ...] ]
	   [ WHERE condition ]
	   [ GROUP BY grouping_element [, ...] ]
	   [ HAVING condition [, ...] ]
	   [ WINDOW window_name AS ( window_definition ) [, ...] ]
	   [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ]
	   [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
	   [ LIMIT { count | ALL } ]
	   [ OFFSET start [ ROW | ROWS ] ]
	   [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
	   [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]

       where from_item can be one of:

	   [ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
		       [ TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ] ]
	   [ LATERAL ] ( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
	   with_query_name [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
	   [ LATERAL ] function_name ( [ argument [, ...] ] )
		       [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
	   [ LATERAL ] function_name ( [ argument [, ...] ] ) [ AS ] alias ( column_definition [, ...] )
	   [ LATERAL ] function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
	   [ LATERAL ] ROWS FROM( function_name ( [ argument [, ...] ] ) [ AS ( column_definition [, ...] ) ] [, ...] )
		       [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
	   from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]

       and grouping_element can be one of:

	   ( )
	   expression
	   ( expression [, ...] )
	   ROLLUP ( { expression | ( expression [, ...] ) } [, ...] )
	   CUBE ( { expression | ( expression [, ...] ) } [, ...] )
	   GROUPING SETS ( grouping_element [, ...] )

       and with_query is:

	   with_query_name [ ( column_name [, ...] ) ] AS ( select | values | insert | update | delete )

       TABLE [ ONLY ] table_name [ * ]

DESCRIPTION
       SELECT retrieves rows from zero or more tables. The general processing
       of SELECT is as follows:

	1. All queries in the WITH list are computed. These effectively serve
	   as temporary tables that can be referenced in the FROM list. A WITH
	   query that is referenced more than once in FROM is computed only
	   once. (See WITH Clause below.)

	2. All elements in the FROM list are computed. (Each element in the
	   FROM list is a real or virtual table.) If more than one element is
	   specified in the FROM list, they are cross-joined together. (See
	   FROM Clause below.)

	3. If the WHERE clause is specified, all rows that do not satisfy the
	   condition are eliminated from the output. (See WHERE Clause below.)

	4. If the GROUP BY clause is specified, or if there are aggregate
	   function calls, the output is combined into groups of rows that
	   match on one or more values, and the results of aggregate functions
	   are computed. If the HAVING clause is present, it eliminates groups
	   that do not satisfy the given condition. (See GROUP BY Clause and
	   HAVING Clause below.)

	5. The actual output rows are computed using the SELECT output
	   expressions for each selected row or row group. (See SELECT List
	   below.)

	6. SELECT DISTINCT eliminates duplicate rows from the result.  SELECT
	   DISTINCT ON eliminates rows that match on all the specified
	   expressions.	 SELECT ALL (the default) will return all candidate
	   rows, including duplicates. (See DISTINCT Clause below.)

	7. Using the operators UNION, INTERSECT, and EXCEPT, the output of
	   more than one SELECT statement can be combined to form a single
	   result set. The UNION operator returns all rows that are in one or
	   both of the result sets. The INTERSECT operator returns all rows
	   that are strictly in both result sets. The EXCEPT operator returns
	   the rows that are in the first result set but not in the second. In
	   all three cases, duplicate rows are eliminated unless ALL is
	   specified. The noise word DISTINCT can be added to explicitly
	   specify eliminating duplicate rows. Notice that DISTINCT is the
	   default behavior here, even though ALL is the default for SELECT
	   itself. (See UNION Clause, INTERSECT Clause, and EXCEPT Clause
	   below.)

	8. If the ORDER BY clause is specified, the returned rows are sorted
	   in the specified order. If ORDER BY is not given, the rows are
	   returned in whatever order the system finds fastest to produce.
	   (See ORDER BY Clause below.)

	9. If the LIMIT (or FETCH FIRST) or OFFSET clause is specified, the
	   SELECT statement only returns a subset of the result rows. (See
	   LIMIT Clause below.)

       10. If FOR UPDATE, FOR NO KEY UPDATE, FOR SHARE or FOR KEY SHARE is
	   specified, the SELECT statement locks the selected rows against
	   concurrent updates. (See The Locking Clause below.)

       You must have SELECT privilege on each column used in a SELECT command.
       The use of FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE or FOR KEY SHARE
       requires UPDATE privilege as well (for at least one column of each
       table so selected).

PARAMETERS
   WITH Clause
       The WITH clause allows you to specify one or more subqueries that can
       be referenced by name in the primary query. The subqueries effectively
       act as temporary tables or views for the duration of the primary query.
       Each subquery can be a SELECT, TABLE, VALUES, INSERT, UPDATE or DELETE
       statement. When writing a data-modifying statement (INSERT, UPDATE or
       DELETE) in WITH, it is usual to include a RETURNING clause. It is the
       output of RETURNING, not the underlying table that the statement
       modifies, that forms the temporary table that is read by the primary
       query. If RETURNING is omitted, the statement is still executed, but it
       produces no output so it cannot be referenced as a table by the primary
       query.

       A name (without schema qualification) must be specified for each WITH
       query. Optionally, a list of column names can be specified; if this is
       omitted, the column names are inferred from the subquery.

       If RECURSIVE is specified, it allows a SELECT subquery to reference
       itself by name. Such a subquery must have the form

	   non_recursive_term UNION [ ALL | DISTINCT ] recursive_term

       where the recursive self-reference must appear on the right-hand side
       of the UNION. Only one recursive self-reference is permitted per query.
       Recursive data-modifying statements are not supported, but you can use
       the results of a recursive SELECT query in a data-modifying statement.
       See Section 7.8 for an example.

       Another effect of RECURSIVE is that WITH queries need not be ordered: a
       query can reference another one that is later in the list. (However,
       circular references, or mutual recursion, are not implemented.) Without
       RECURSIVE, WITH queries can only reference sibling WITH queries that
       are earlier in the WITH list.

       A key property of WITH queries is that they are evaluated only once per
       execution of the primary query, even if the primary query refers to
       them more than once. In particular, data-modifying statements are
       guaranteed to be executed once and only once, regardless of whether the
       primary query reads all or any of their output.

       The primary query and the WITH queries are all (notionally) executed at
       the same time. This implies that the effects of a data-modifying
       statement in WITH cannot be seen from other parts of the query, other
       than by reading its RETURNING output. If two such data-modifying
       statements attempt to modify the same row, the results are unspecified.

       See Section 7.8 for additional information.

   FROM Clause
       The FROM clause specifies one or more source tables for the SELECT. If
       multiple sources are specified, the result is the Cartesian product
       (cross join) of all the sources. But usually qualification conditions
       are added (via WHERE) to restrict the returned rows to a small subset
       of the Cartesian product.

       The FROM clause can contain the following elements:

       table_name
	   The name (optionally schema-qualified) of an existing table or
	   view. If ONLY is specified before the table name, only that table
	   is scanned. If ONLY is not specified, the table and all its
	   descendant tables (if any) are scanned. Optionally, * can be
	   specified after the table name to explicitly indicate that
	   descendant tables are included.

       alias
	   A substitute name for the FROM item containing the alias. An alias
	   is used for brevity or to eliminate ambiguity for self-joins (where
	   the same table is scanned multiple times). When an alias is
	   provided, it completely hides the actual name of the table or
	   function; for example given FROM foo AS f, the remainder of the
	   SELECT must refer to this FROM item as f not foo. If an alias is
	   written, a column alias list can also be written to provide
	   substitute names for one or more columns of the table.

       TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed )
       ]
	   A TABLESAMPLE clause after a table_name indicates that the
	   specified sampling_method should be used to retrieve a subset of
	   the rows in that table. This sampling precedes the application of
	   any other filters such as WHERE clauses. The standard PostgreSQL
	   distribution includes two sampling methods, BERNOULLI and SYSTEM,
	   and other sampling methods can be installed in the database via
	   extensions.

	   The BERNOULLI and SYSTEM sampling methods each accept a single
	   argument which is the fraction of the table to sample, expressed as
	   a percentage between 0 and 100. This argument can be any
	   real-valued expression. (Other sampling methods might accept more
	   or different arguments.) These two methods each return a
	   randomly-chosen sample of the table that will contain approximately
	   the specified percentage of the table's rows. The BERNOULLI method
	   scans the whole table and selects or ignores individual rows
	   independently with the specified probability. The SYSTEM method
	   does block-level sampling with each block having the specified
	   chance of being selected; all rows in each selected block are
	   returned. The SYSTEM method is significantly faster than the
	   BERNOULLI method when small sampling percentages are specified, but
	   it may return a less-random sample of the table as a result of
	   clustering effects.

	   The optional REPEATABLE clause specifies a seed number or
	   expression to use for generating random numbers within the sampling
	   method. The seed value can be any non-null floating-point value.
	   Two queries that specify the same seed and argument values will
	   select the same sample of the table, if the table has not been
	   changed meanwhile. But different seed values will usually produce
	   different samples. If REPEATABLE is not given then a new random
	   sample is selected for each query, based upon a system-generated
	   seed. Note that some add-on sampling methods do not accept
	   REPEATABLE, and will always produce new samples on each use.

       select
	   A sub-SELECT can appear in the FROM clause. This acts as though its
	   output were created as a temporary table for the duration of this
	   single SELECT command. Note that the sub-SELECT must be surrounded
	   by parentheses, and an alias must be provided for it. A VALUES(7)
	   command can also be used here.

       with_query_name
	   A WITH query is referenced by writing its name, just as though the
	   query's name were a table name. (In fact, the WITH query hides any
	   real table of the same name for the purposes of the primary query.
	   If necessary, you can refer to a real table of the same name by
	   schema-qualifying the table's name.) An alias can be provided in
	   the same way as for a table.

       function_name
	   Function calls can appear in the FROM clause. (This is especially
	   useful for functions that return result sets, but any function can
	   be used.) This acts as though the function's output were created as
	   a temporary table for the duration of this single SELECT command.
	   When the optional WITH ORDINALITY clause is added to the function
	   call, a new column is appended after all the function's output
	   columns with numbering for each row.

	   An alias can be provided in the same way as for a table. If an
	   alias is written, a column alias list can also be written to
	   provide substitute names for one or more attributes of the
	   function's composite return type, including the column added by
	   ORDINALITY if present.

	   Multiple function calls can be combined into a single FROM-clause
	   item by surrounding them with ROWS FROM( ... ). The output of such
	   an item is the concatenation of the first row from each function,
	   then the second row from each function, etc. If some of the
	   functions produce fewer rows than others, null values are
	   substituted for the missing data, so that the total number of rows
	   returned is always the same as for the function that produced the
	   most rows.

	   If the function has been defined as returning the record data type,
	   then an alias or the key word AS must be present, followed by a
	   column definition list in the form ( column_name data_type [, ...
	   ]). The column definition list must match the actual number and
	   types of columns returned by the function.

	   When using the ROWS FROM( ... ) syntax, if one of the functions
	   requires a column definition list, it's preferred to put the column
	   definition list after the function call inside ROWS FROM( ... ). A
	   column definition list can be placed after the ROWS FROM( ... )
	   construct only if there's just a single function and no WITH
	   ORDINALITY clause.

	   To use ORDINALITY together with a column definition list, you must
	   use the ROWS FROM( ... ) syntax and put the column definition list
	   inside ROWS FROM( ... ).

       join_type
	   One of

	   ·   [ INNER ] JOIN

	   ·   LEFT [ OUTER ] JOIN

	   ·   RIGHT [ OUTER ] JOIN

	   ·   FULL [ OUTER ] JOIN

	   ·   CROSS JOIN

	   For the INNER and OUTER join types, a join condition must be
	   specified, namely exactly one of NATURAL, ON join_condition, or
	   USING (join_column [, ...]). See below for the meaning. For CROSS
	   JOIN, none of these clauses can appear.

	   A JOIN clause combines two FROM items, which for convenience we
	   will refer to as “tables”, though in reality they can be any type
	   of FROM item. Use parentheses if necessary to determine the order
	   of nesting. In the absence of parentheses, JOINs nest
	   left-to-right. In any case JOIN binds more tightly than the commas
	   separating FROM-list items.

	   CROSS JOIN and INNER JOIN produce a simple Cartesian product, the
	   same result as you get from listing the two tables at the top level
	   of FROM, but restricted by the join condition (if any).  CROSS JOIN
	   is equivalent to INNER JOIN ON (TRUE), that is, no rows are removed
	   by qualification. These join types are just a notational
	   convenience, since they do nothing you couldn't do with plain FROM
	   and WHERE.

	   LEFT OUTER JOIN returns all rows in the qualified Cartesian product
	   (i.e., all combined rows that pass its join condition), plus one
	   copy of each row in the left-hand table for which there was no
	   right-hand row that passed the join condition. This left-hand row
	   is extended to the full width of the joined table by inserting null
	   values for the right-hand columns. Note that only the JOIN clause's
	   own condition is considered while deciding which rows have matches.
	   Outer conditions are applied afterwards.

	   Conversely, RIGHT OUTER JOIN returns all the joined rows, plus one
	   row for each unmatched right-hand row (extended with nulls on the
	   left). This is just a notational convenience, since you could
	   convert it to a LEFT OUTER JOIN by switching the left and right
	   tables.

	   FULL OUTER JOIN returns all the joined rows, plus one row for each
	   unmatched left-hand row (extended with nulls on the right), plus
	   one row for each unmatched right-hand row (extended with nulls on
	   the left).

       ON join_condition
	   join_condition is an expression resulting in a value of type
	   boolean (similar to a WHERE clause) that specifies which rows in a
	   join are considered to match.

       USING ( join_column [, ...] )
	   A clause of the form USING ( a, b, ... ) is shorthand for ON
	   left_table.a = right_table.a AND left_table.b = right_table.b ....
	   Also, USING implies that only one of each pair of equivalent
	   columns will be included in the join output, not both.

       NATURAL
	   NATURAL is shorthand for a USING list that mentions all columns in
	   the two tables that have matching names. If there are no common
	   column names, NATURAL is equivalent to ON TRUE.

       LATERAL
	   The LATERAL key word can precede a sub-SELECT FROM item. This
	   allows the sub-SELECT to refer to columns of FROM items that appear
	   before it in the FROM list. (Without LATERAL, each sub-SELECT is
	   evaluated independently and so cannot cross-reference any other
	   FROM item.)

	   LATERAL can also precede a function-call FROM item, but in this
	   case it is a noise word, because the function expression can refer
	   to earlier FROM items in any case.

	   A LATERAL item can appear at top level in the FROM list, or within
	   a JOIN tree. In the latter case it can also refer to any items that
	   are on the left-hand side of a JOIN that it is on the right-hand
	   side of.

	   When a FROM item contains LATERAL cross-references, evaluation
	   proceeds as follows: for each row of the FROM item providing the
	   cross-referenced column(s), or set of rows of multiple FROM items
	   providing the columns, the LATERAL item is evaluated using that row
	   or row set's values of the columns. The resulting row(s) are joined
	   as usual with the rows they were computed from. This is repeated
	   for each row or set of rows from the column source table(s).

	   The column source table(s) must be INNER or LEFT joined to the
	   LATERAL item, else there would not be a well-defined set of rows
	   from which to compute each set of rows for the LATERAL item. Thus,
	   although a construct such as X RIGHT JOIN LATERAL Y is
	   syntactically valid, it is not actually allowed for Y to reference
	   X.

   WHERE Clause
       The optional WHERE clause has the general form

	   WHERE condition

       where condition is any expression that evaluates to a result of type
       boolean. Any row that does not satisfy this condition will be
       eliminated from the output. A row satisfies the condition if it returns
       true when the actual row values are substituted for any variable
       references.

   GROUP BY Clause
       The optional GROUP BY clause has the general form

	   GROUP BY grouping_element [, ...]

       GROUP BY will condense into a single row all selected rows that share
       the same values for the grouped expressions. An expression used inside
       a grouping_element can be an input column name, or the name or ordinal
       number of an output column (SELECT list item), or an arbitrary
       expression formed from input-column values. In case of ambiguity, a
       GROUP BY name will be interpreted as an input-column name rather than
       an output column name.

       If any of GROUPING SETS, ROLLUP or CUBE are present as grouping
       elements, then the GROUP BY clause as a whole defines some number of
       independent grouping sets. The effect of this is equivalent to
       constructing a UNION ALL between subqueries with the individual
       grouping sets as their GROUP BY clauses. For further details on the
       handling of grouping sets see Section 7.2.4.

       Aggregate functions, if any are used, are computed across all rows
       making up each group, producing a separate value for each group. (If
       there are aggregate functions but no GROUP BY clause, the query is
       treated as having a single group comprising all the selected rows.) The
       set of rows fed to each aggregate function can be further filtered by
       attaching a FILTER clause to the aggregate function call; see
       Section 4.2.7 for more information. When a FILTER clause is present,
       only those rows matching it are included in the input to that aggregate
       function.

       When GROUP BY is present, or any aggregate functions are present, it is
       not valid for the SELECT list expressions to refer to ungrouped columns
       except within aggregate functions or when the ungrouped column is
       functionally dependent on the grouped columns, since there would
       otherwise be more than one possible value to return for an ungrouped
       column. A functional dependency exists if the grouped columns (or a
       subset thereof) are the primary key of the table containing the
       ungrouped column.

       Keep in mind that all aggregate functions are evaluated before
       evaluating any “scalar” expressions in the HAVING clause or SELECT
       list. This means that, for example, a CASE expression cannot be used to
       skip evaluation of an aggregate function; see Section 4.2.14.

       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
       cannot be specified with GROUP BY.

   HAVING Clause
       The optional HAVING clause has the general form

	   HAVING condition

       where condition is the same as specified for the WHERE clause.

       HAVING eliminates group rows that do not satisfy the condition.	HAVING
       is different from WHERE: WHERE filters individual rows before the
       application of GROUP BY, while HAVING filters group rows created by
       GROUP BY. Each column referenced in condition must unambiguously
       reference a grouping column, unless the reference appears within an
       aggregate function or the ungrouped column is functionally dependent on
       the grouping columns.

       The presence of HAVING turns a query into a grouped query even if there
       is no GROUP BY clause. This is the same as what happens when the query
       contains aggregate functions but no GROUP BY clause. All the selected
       rows are considered to form a single group, and the SELECT list and
       HAVING clause can only reference table columns from within aggregate
       functions. Such a query will emit a single row if the HAVING condition
       is true, zero rows if it is not true.

       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
       cannot be specified with HAVING.

   WINDOW Clause
       The optional WINDOW clause has the general form

	   WINDOW window_name AS ( window_definition ) [, ...]

       where window_name is a name that can be referenced from OVER clauses or
       subsequent window definitions, and window_definition is

	   [ existing_window_name ]
	   [ PARTITION BY expression [, ...] ]
	   [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
	   [ frame_clause ]

       If an existing_window_name is specified it must refer to an earlier
       entry in the WINDOW list; the new window copies its partitioning clause
       from that entry, as well as its ordering clause if any. In this case
       the new window cannot specify its own PARTITION BY clause, and it can
       specify ORDER BY only if the copied window does not have one. The new
       window always uses its own frame clause; the copied window must not
       specify a frame clause.

       The elements of the PARTITION BY list are interpreted in much the same
       fashion as elements of a GROUP BY Clause, except that they are always
       simple expressions and never the name or number of an output column.
       Another difference is that these expressions can contain aggregate
       function calls, which are not allowed in a regular GROUP BY clause.
       They are allowed here because windowing occurs after grouping and
       aggregation.

       Similarly, the elements of the ORDER BY list are interpreted in much
       the same fashion as elements of an ORDER BY Clause, except that the
       expressions are always taken as simple expressions and never the name
       or number of an output column.

       The optional frame_clause defines the window frame for window functions
       that depend on the frame (not all do). The window frame is a set of
       related rows for each row of the query (called the current row). The
       frame_clause can be one of

	   { RANGE | ROWS } frame_start
	   { RANGE | ROWS } BETWEEN frame_start AND frame_end

       where frame_start and frame_end can be one of

	   UNBOUNDED PRECEDING
	   value PRECEDING
	   CURRENT ROW
	   value FOLLOWING
	   UNBOUNDED FOLLOWING

       If frame_end is omitted it defaults to CURRENT ROW. Restrictions are
       that frame_start cannot be UNBOUNDED FOLLOWING, frame_end cannot be
       UNBOUNDED PRECEDING, and the frame_end choice cannot appear earlier in
       the above list than the frame_start choice — for example RANGE BETWEEN
       CURRENT ROW AND value PRECEDING is not allowed.

       The default framing option is RANGE UNBOUNDED PRECEDING, which is the
       same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it sets the
       frame to be all rows from the partition start up through the current
       row's last peer (a row that ORDER BY considers equivalent to the
       current row, or all rows if there is no ORDER BY). In general,
       UNBOUNDED PRECEDING means that the frame starts with the first row of
       the partition, and similarly UNBOUNDED FOLLOWING means that the frame
       ends with the last row of the partition (regardless of RANGE or ROWS
       mode). In ROWS mode, CURRENT ROW means that the frame starts or ends
       with the current row; but in RANGE mode it means that the frame starts
       or ends with the current row's first or last peer in the ORDER BY
       ordering. The value PRECEDING and value FOLLOWING cases are currently
       only allowed in ROWS mode. They indicate that the frame starts or ends
       with the row that many rows before or after the current row.  value
       must be an integer expression not containing any variables, aggregate
       functions, or window functions. The value must not be null or negative;
       but it can be zero, which selects the current row itself.

       Beware that the ROWS options can produce unpredictable results if the
       ORDER BY ordering does not order the rows uniquely. The RANGE options
       are designed to ensure that rows that are peers in the ORDER BY
       ordering are treated alike; all peer rows will be in the same frame.

       The purpose of a WINDOW clause is to specify the behavior of window
       functions appearing in the query's SELECT List or ORDER BY Clause.
       These functions can reference the WINDOW clause entries by name in
       their OVER clauses. A WINDOW clause entry does not have to be
       referenced anywhere, however; if it is not used in the query it is
       simply ignored. It is possible to use window functions without any
       WINDOW clause at all, since a window function call can specify its
       window definition directly in its OVER clause. However, the WINDOW
       clause saves typing when the same window definition is needed for more
       than one window function.

       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
       cannot be specified with WINDOW.

       Window functions are described in detail in Section 3.5, Section 4.2.8,
       and Section 7.2.5.

   SELECT List
       The SELECT list (between the key words SELECT and FROM) specifies
       expressions that form the output rows of the SELECT statement. The
       expressions can (and usually do) refer to columns computed in the FROM
       clause.

       Just as in a table, every output column of a SELECT has a name. In a
       simple SELECT this name is just used to label the column for display,
       but when the SELECT is a sub-query of a larger query, the name is seen
       by the larger query as the column name of the virtual table produced by
       the sub-query. To specify the name to use for an output column, write
       AS output_name after the column's expression. (You can omit AS, but
       only if the desired output name does not match any PostgreSQL keyword
       (see Appendix C). For protection against possible future keyword
       additions, it is recommended that you always either write AS or
       double-quote the output name.) If you do not specify a column name, a
       name is chosen automatically by PostgreSQL. If the column's expression
       is a simple column reference then the chosen name is the same as that
       column's name. In more complex cases a function or type name may be
       used, or the system may fall back on a generated name such as ?column?.

       An output column's name can be used to refer to the column's value in
       ORDER BY and GROUP BY clauses, but not in the WHERE or HAVING clauses;
       there you must write out the expression instead.

       Instead of an expression, * can be written in the output list as a
       shorthand for all the columns of the selected rows. Also, you can write
       table_name.*  as a shorthand for the columns coming from just that
       table. In these cases it is not possible to specify new names with AS;
       the output column names will be the same as the table columns' names.

       According to the SQL standard, the expressions in the output list
       should be computed before applying DISTINCT, ORDER BY, or LIMIT. This
       is obviously necessary when using DISTINCT, since otherwise it's not
       clear what values are being made distinct. However, in many cases it is
       convenient if output expressions are computed after ORDER BY and LIMIT;
       particularly if the output list contains any volatile or expensive
       functions. With that behavior, the order of function evaluations is
       more intuitive and there will not be evaluations corresponding to rows
       that never appear in the output.	 PostgreSQL will effectively evaluate
       output expressions after sorting and limiting, so long as those
       expressions are not referenced in DISTINCT, ORDER BY or GROUP BY. (As a
       counterexample, SELECT f(x) FROM tab ORDER BY 1 clearly must evaluate
       f(x) before sorting.) Output expressions that contain set-returning
       functions are effectively evaluated after sorting and before limiting,
       so that LIMIT will act to cut off the output from a set-returning
       function.

	   Note
	   PostgreSQL versions before 9.6 did not provide any guarantees about
	   the timing of evaluation of output expressions versus sorting and
	   limiting; it depended on the form of the chosen query plan.

   DISTINCT Clause
       If SELECT DISTINCT is specified, all duplicate rows are removed from
       the result set (one row is kept from each group of duplicates).	SELECT
       ALL specifies the opposite: all rows are kept; that is the default.

       SELECT DISTINCT ON ( expression [, ...] ) keeps only the first row of
       each set of rows where the given expressions evaluate to equal. The
       DISTINCT ON expressions are interpreted using the same rules as for
       ORDER BY (see above). Note that the “first row” of each set is
       unpredictable unless ORDER BY is used to ensure that the desired row
       appears first. For example:

	   SELECT DISTINCT ON (location) location, time, report
	       FROM weather_reports
	       ORDER BY location, time DESC;

       retrieves the most recent weather report for each location. But if we
       had not used ORDER BY to force descending order of time values for each
       location, we'd have gotten a report from an unpredictable time for each
       location.

       The DISTINCT ON expression(s) must match the leftmost ORDER BY
       expression(s). The ORDER BY clause will normally contain additional
       expression(s) that determine the desired precedence of rows within each
       DISTINCT ON group.

       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
       cannot be specified with DISTINCT.

   UNION Clause
       The UNION clause has this general form:

	   select_statement UNION [ ALL | DISTINCT ] select_statement

       select_statement is any SELECT statement without an ORDER BY, LIMIT,
       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.
       (ORDER BY and LIMIT can be attached to a subexpression if it is
       enclosed in parentheses. Without parentheses, these clauses will be
       taken to apply to the result of the UNION, not to its right-hand input
       expression.)

       The UNION operator computes the set union of the rows returned by the
       involved SELECT statements. A row is in the set union of two result
       sets if it appears in at least one of the result sets. The two SELECT
       statements that represent the direct operands of the UNION must produce
       the same number of columns, and corresponding columns must be of
       compatible data types.

       The result of UNION does not contain any duplicate rows unless the ALL
       option is specified.  ALL prevents elimination of duplicates.
       (Therefore, UNION ALL is usually significantly quicker than UNION; use
       ALL when you can.)  DISTINCT can be written to explicitly specify the
       default behavior of eliminating duplicate rows.

       Multiple UNION operators in the same SELECT statement are evaluated
       left to right, unless otherwise indicated by parentheses.

       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
       cannot be specified either for a UNION result or for any input of a
       UNION.

   INTERSECT Clause
       The INTERSECT clause has this general form:

	   select_statement INTERSECT [ ALL | DISTINCT ] select_statement

       select_statement is any SELECT statement without an ORDER BY, LIMIT,
       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.

       The INTERSECT operator computes the set intersection of the rows
       returned by the involved SELECT statements. A row is in the
       intersection of two result sets if it appears in both result sets.

       The result of INTERSECT does not contain any duplicate rows unless the
       ALL option is specified. With ALL, a row that has m duplicates in the
       left table and n duplicates in the right table will appear min(m,n)
       times in the result set.	 DISTINCT can be written to explicitly specify
       the default behavior of eliminating duplicate rows.

       Multiple INTERSECT operators in the same SELECT statement are evaluated
       left to right, unless parentheses dictate otherwise.  INTERSECT binds
       more tightly than UNION. That is, A UNION B INTERSECT C will be read as
       A UNION (B INTERSECT C).

       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
       cannot be specified either for an INTERSECT result or for any input of
       an INTERSECT.

   EXCEPT Clause
       The EXCEPT clause has this general form:

	   select_statement EXCEPT [ ALL | DISTINCT ] select_statement

       select_statement is any SELECT statement without an ORDER BY, LIMIT,
       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.

       The EXCEPT operator computes the set of rows that are in the result of
       the left SELECT statement but not in the result of the right one.

       The result of EXCEPT does not contain any duplicate rows unless the ALL
       option is specified. With ALL, a row that has m duplicates in the left
       table and n duplicates in the right table will appear max(m-n,0) times
       in the result set.  DISTINCT can be written to explicitly specify the
       default behavior of eliminating duplicate rows.

       Multiple EXCEPT operators in the same SELECT statement are evaluated
       left to right, unless parentheses dictate otherwise.  EXCEPT binds at
       the same level as UNION.

       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
       cannot be specified either for an EXCEPT result or for any input of an
       EXCEPT.

   ORDER BY Clause
       The optional ORDER BY clause has this general form:

	   ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...]

       The ORDER BY clause causes the result rows to be sorted according to
       the specified expression(s). If two rows are equal according to the
       leftmost expression, they are compared according to the next expression
       and so on. If they are equal according to all specified expressions,
       they are returned in an implementation-dependent order.

       Each expression can be the name or ordinal number of an output column
       (SELECT list item), or it can be an arbitrary expression formed from
       input-column values.

       The ordinal number refers to the ordinal (left-to-right) position of
       the output column. This feature makes it possible to define an ordering
       on the basis of a column that does not have a unique name. This is
       never absolutely necessary because it is always possible to assign a
       name to an output column using the AS clause.

       It is also possible to use arbitrary expressions in the ORDER BY
       clause, including columns that do not appear in the SELECT output list.
       Thus the following statement is valid:

	   SELECT name FROM distributors ORDER BY code;

       A limitation of this feature is that an ORDER BY clause applying to the
       result of a UNION, INTERSECT, or EXCEPT clause can only specify an
       output column name or number, not an expression.

       If an ORDER BY expression is a simple name that matches both an output
       column name and an input column name, ORDER BY will interpret it as the
       output column name. This is the opposite of the choice that GROUP BY
       will make in the same situation. This inconsistency is made to be
       compatible with the SQL standard.

       Optionally one can add the key word ASC (ascending) or DESC
       (descending) after any expression in the ORDER BY clause. If not
       specified, ASC is assumed by default. Alternatively, a specific
       ordering operator name can be specified in the USING clause. An
       ordering operator must be a less-than or greater-than member of some
       B-tree operator family.	ASC is usually equivalent to USING < and DESC
       is usually equivalent to USING >. (But the creator of a user-defined
       data type can define exactly what the default sort ordering is, and it
       might correspond to operators with other names.)

       If NULLS LAST is specified, null values sort after all non-null values;
       if NULLS FIRST is specified, null values sort before all non-null
       values. If neither is specified, the default behavior is NULLS LAST
       when ASC is specified or implied, and NULLS FIRST when DESC is
       specified (thus, the default is to act as though nulls are larger than
       non-nulls). When USING is specified, the default nulls ordering depends
       on whether the operator is a less-than or greater-than operator.

       Note that ordering options apply only to the expression they follow;
       for example ORDER BY x, y DESC does not mean the same thing as ORDER BY
       x DESC, y DESC.

       Character-string data is sorted according to the collation that applies
       to the column being sorted. That can be overridden at need by including
       a COLLATE clause in the expression, for example ORDER BY mycolumn
       COLLATE "en_US". For more information see Section 4.2.10 and
       Section 23.2.

   LIMIT Clause
       The LIMIT clause consists of two independent sub-clauses:

	   LIMIT { count | ALL }
	   OFFSET start

       count specifies the maximum number of rows to return, while start
       specifies the number of rows to skip before starting to return rows.
       When both are specified, start rows are skipped before starting to
       count the count rows to be returned.

       If the count expression evaluates to NULL, it is treated as LIMIT ALL,
       i.e., no limit. If start evaluates to NULL, it is treated the same as
       OFFSET 0.

       SQL:2008 introduced a different syntax to achieve the same result,
       which PostgreSQL also supports. It is:

	   OFFSET start { ROW | ROWS }
	   FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY

       In this syntax, to write anything except a simple integer constant for
       start or count, you must write parentheses around it. If count is
       omitted in a FETCH clause, it defaults to 1.  ROW and ROWS as well as
       FIRST and NEXT are noise words that don't influence the effects of
       these clauses. According to the standard, the OFFSET clause must come
       before the FETCH clause if both are present; but PostgreSQL is laxer
       and allows either order.

       When using LIMIT, it is a good idea to use an ORDER BY clause that
       constrains the result rows into a unique order. Otherwise you will get
       an unpredictable subset of the query's rows — you might be asking for
       the tenth through twentieth rows, but tenth through twentieth in what
       ordering? You don't know what ordering unless you specify ORDER BY.

       The query planner takes LIMIT into account when generating a query
       plan, so you are very likely to get different plans (yielding different
       row orders) depending on what you use for LIMIT and OFFSET. Thus, using
       different LIMIT/OFFSET values to select different subsets of a query
       result will give inconsistent results unless you enforce a predictable
       result ordering with ORDER BY. This is not a bug; it is an inherent
       consequence of the fact that SQL does not promise to deliver the
       results of a query in any particular order unless ORDER BY is used to
       constrain the order.

       It is even possible for repeated executions of the same LIMIT query to
       return different subsets of the rows of a table, if there is not an
       ORDER BY to enforce selection of a deterministic subset. Again, this is
       not a bug; determinism of the results is simply not guaranteed in such
       a case.

   The Locking Clause
       FOR UPDATE, FOR NO KEY UPDATE, FOR SHARE and FOR KEY SHARE are locking
       clauses; they affect how SELECT locks rows as they are obtained from
       the table.

       The locking clause has the general form

	   FOR lock_strength [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ]

       where lock_strength can be one of

	   UPDATE
	   NO KEY UPDATE
	   SHARE
	   KEY SHARE

       For more information on each row-level lock mode, refer to
       Section 13.3.2.

       To prevent the operation from waiting for other transactions to commit,
       use either the NOWAIT or SKIP LOCKED option. With NOWAIT, the statement
       reports an error, rather than waiting, if a selected row cannot be
       locked immediately. With SKIP LOCKED, any selected rows that cannot be
       immediately locked are skipped. Skipping locked rows provides an
       inconsistent view of the data, so this is not suitable for general
       purpose work, but can be used to avoid lock contention with multiple
       consumers accessing a queue-like table. Note that NOWAIT and SKIP
       LOCKED apply only to the row-level lock(s) — the required ROW SHARE
       table-level lock is still taken in the ordinary way (see Chapter 13).
       You can use LOCK(7) with the NOWAIT option first, if you need to
       acquire the table-level lock without waiting.

       If specific tables are named in a locking clause, then only rows coming
       from those tables are locked; any other tables used in the SELECT are
       simply read as usual. A locking clause without a table list affects all
       tables used in the statement. If a locking clause is applied to a view
       or sub-query, it affects all tables used in the view or sub-query.
       However, these clauses do not apply to WITH queries referenced by the
       primary query. If you want row locking to occur within a WITH query,
       specify a locking clause within the WITH query.

       Multiple locking clauses can be written if it is necessary to specify
       different locking behavior for different tables. If the same table is
       mentioned (or implicitly affected) by more than one locking clause,
       then it is processed as if it was only specified by the strongest one.
       Similarly, a table is processed as NOWAIT if that is specified in any
       of the clauses affecting it. Otherwise, it is processed as SKIP LOCKED
       if that is specified in any of the clauses affecting it.

       The locking clauses cannot be used in contexts where returned rows
       cannot be clearly identified with individual table rows; for example
       they cannot be used with aggregation.

       When a locking clause appears at the top level of a SELECT query, the
       rows that are locked are exactly those that are returned by the query;
       in the case of a join query, the rows locked are those that contribute
       to returned join rows. In addition, rows that satisfied the query
       conditions as of the query snapshot will be locked, although they will
       not be returned if they were updated after the snapshot and no longer
       satisfy the query conditions. If a LIMIT is used, locking stops once
       enough rows have been returned to satisfy the limit (but note that rows
       skipped over by OFFSET will get locked). Similarly, if a locking clause
       is used in a cursor's query, only rows actually fetched or stepped past
       by the cursor will be locked.

       When a locking clause appears in a sub-SELECT, the rows locked are
       those returned to the outer query by the sub-query. This might involve
       fewer rows than inspection of the sub-query alone would suggest, since
       conditions from the outer query might be used to optimize execution of
       the sub-query. For example,

	   SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;

       will lock only rows having col1 = 5, even though that condition is not
       textually within the sub-query.

       Previous releases failed to preserve a lock which is upgraded by a
       later savepoint. For example, this code:

	   BEGIN;
	   SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
	   SAVEPOINT s;
	   UPDATE mytable SET ... WHERE key = 1;
	   ROLLBACK TO s;

       would fail to preserve the FOR UPDATE lock after the ROLLBACK TO. This
       has been fixed in release 9.3.

	   Caution
	   It is possible for a SELECT command running at the READ COMMITTED
	   transaction isolation level and using ORDER BY and a locking clause
	   to return rows out of order. This is because ORDER BY is applied
	   first. The command sorts the result, but might then block trying to
	   obtain a lock on one or more of the rows. Once the SELECT unblocks,
	   some of the ordering column values might have been modified,
	   leading to those rows appearing to be out of order (though they are
	   in order in terms of the original column values). This can be
	   worked around at need by placing the FOR UPDATE/SHARE clause in a
	   sub-query, for example

	       SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;

	   Note that this will result in locking all rows of mytable, whereas
	   FOR UPDATE at the top level would lock only the actually returned
	   rows. This can make for a significant performance difference,
	   particularly if the ORDER BY is combined with LIMIT or other
	   restrictions. So this technique is recommended only if concurrent
	   updates of the ordering columns are expected and a strictly sorted
	   result is required.

	   At the REPEATABLE READ or SERIALIZABLE transaction isolation level
	   this would cause a serialization failure (with a SQLSTATE of
	   '40001'), so there is no possibility of receiving rows out of order
	   under these isolation levels.

   TABLE Command
       The command

	   TABLE name

       is equivalent to

	   SELECT * FROM name

       It can be used as a top-level command or as a space-saving syntax
       variant in parts of complex queries. Only the WITH, UNION, INTERSECT,
       EXCEPT, ORDER BY, LIMIT, OFFSET, FETCH and FOR locking clauses can be
       used with TABLE; the WHERE clause and any form of aggregation cannot be
       used.

EXAMPLES
       To join the table films with the table distributors:

	   SELECT f.title, f.did, d.name, f.date_prod, f.kind
	       FROM distributors d, films f
	       WHERE f.did = d.did

		  title	      | did |	  name	   | date_prod	|   kind
	   -------------------+-----+--------------+------------+----------
	    The Third Man     | 101 | British Lion | 1949-12-23 | Drama
	    The African Queen | 101 | British Lion | 1951-08-11 | Romantic
	    ...

       To sum the column len of all films and group the results by kind:

	   SELECT kind, sum(len) AS total FROM films GROUP BY kind;

	      kind   | total
	   ----------+-------
	    Action   | 07:34
	    Comedy   | 02:58
	    Drama    | 14:28
	    Musical  | 06:42
	    Romantic | 04:38

       To sum the column len of all films, group the results by kind and show
       those group totals that are less than 5 hours:

	   SELECT kind, sum(len) AS total
	       FROM films
	       GROUP BY kind
	       HAVING sum(len) < interval '5 hours';

	      kind   | total
	   ----------+-------
	    Comedy   | 02:58
	    Romantic | 04:38

       The following two examples are identical ways of sorting the individual
       results according to the contents of the second column (name):

	   SELECT * FROM distributors ORDER BY name;
	   SELECT * FROM distributors ORDER BY 2;

	    did |	name
	   -----+------------------
	    109 | 20th Century Fox
	    110 | Bavaria Atelier
	    101 | British Lion
	    107 | Columbia
	    102 | Jean Luc Godard
	    113 | Luso films
	    104 | Mosfilm
	    103 | Paramount
	    106 | Toho
	    105 | United Artists
	    111 | Walt Disney
	    112 | Warner Bros.
	    108 | Westward

       The next example shows how to obtain the union of the tables
       distributors and actors, restricting the results to those that begin
       with the letter W in each table. Only distinct rows are wanted, so the
       key word ALL is omitted.

	   distributors:	       actors:
	    did |     name		id |	 name
	   -----+--------------	       ----+----------------
	    108 | Westward		 1 | Woody Allen
	    111 | Walt Disney		 2 | Warren Beatty
	    112 | Warner Bros.		 3 | Walter Matthau
	    ...				...

	   SELECT distributors.name
	       FROM distributors
	       WHERE distributors.name LIKE 'W%'
	   UNION
	   SELECT actors.name
	       FROM actors
	       WHERE actors.name LIKE 'W%';

		 name
	   ----------------
	    Walt Disney
	    Walter Matthau
	    Warner Bros.
	    Warren Beatty
	    Westward
	    Woody Allen

       This example shows how to use a function in the FROM clause, both with
       and without a column definition list:

	   CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
	       SELECT * FROM distributors WHERE did = $1;
	   $$ LANGUAGE SQL;

	   SELECT * FROM distributors(111);
	    did |    name
	   -----+-------------
	    111 | Walt Disney

	   CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
	       SELECT * FROM distributors WHERE did = $1;
	   $$ LANGUAGE SQL;

	   SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
	    f1	|     f2
	   -----+-------------
	    111 | Walt Disney

       Here is an example of a function with an ordinality column added:

	   SELECT * FROM unnest(ARRAY['a','b','c','d','e','f']) WITH ORDINALITY;
	    unnest | ordinality
	   --------+----------
	    a	   |	    1
	    b	   |	    2
	    c	   |	    3
	    d	   |	    4
	    e	   |	    5
	    f	   |	    6
	   (6 rows)

       This example shows how to use a simple WITH clause:

	   WITH t AS (
	       SELECT random() as x FROM generate_series(1, 3)
	     )
	   SELECT * FROM t
	   UNION ALL
	   SELECT * FROM t

		    x
	   --------------------
	     0.534150459803641
	     0.520092216785997
	    0.0735620250925422
	     0.534150459803641
	     0.520092216785997
	    0.0735620250925422

       Notice that the WITH query was evaluated only once, so that we got two
       sets of the same three random values.

       This example uses WITH RECURSIVE to find all subordinates (direct or
       indirect) of the employee Mary, and their level of indirectness, from a
       table that shows only direct subordinates:

	   WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
	       SELECT 1, employee_name, manager_name
	       FROM employee
	       WHERE manager_name = 'Mary'
	     UNION ALL
	       SELECT er.distance + 1, e.employee_name, e.manager_name
	       FROM employee_recursive er, employee e
	       WHERE er.employee_name = e.manager_name
	     )
	   SELECT distance, employee_name FROM employee_recursive;

       Notice the typical form of recursive queries: an initial condition,
       followed by UNION, followed by the recursive part of the query. Be sure
       that the recursive part of the query will eventually return no tuples,
       or else the query will loop indefinitely. (See Section 7.8 for more
       examples.)

       This example uses LATERAL to apply a set-returning function
       get_product_names() for each row of the manufacturers table:

	   SELECT m.name AS mname, pname
	   FROM manufacturers m, LATERAL get_product_names(m.id) pname;

       Manufacturers not currently having any products would not appear in the
       result, since it is an inner join. If we wished to include the names of
       such manufacturers in the result, we could do:

	   SELECT m.name AS mname, pname
	   FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true;

COMPATIBILITY
       Of course, the SELECT statement is compatible with the SQL standard.
       But there are some extensions and some missing features.

   Omitted FROM Clauses
       PostgreSQL allows one to omit the FROM clause. It has a straightforward
       use to compute the results of simple expressions:

	   SELECT 2+2;

	    ?column?
	   ----------
		   4

       Some other SQL databases cannot do this except by introducing a dummy
       one-row table from which to do the SELECT.

       Note that if a FROM clause is not specified, the query cannot reference
       any database tables. For example, the following query is invalid:

	   SELECT distributors.* WHERE distributors.name = 'Westward';

       PostgreSQL releases prior to 8.1 would accept queries of this form, and
       add an implicit entry to the query's FROM clause for each table
       referenced by the query. This is no longer allowed.

   Empty SELECT Lists
       The list of output expressions after SELECT can be empty, producing a
       zero-column result table. This is not valid syntax according to the SQL
       standard.  PostgreSQL allows it to be consistent with allowing
       zero-column tables. However, an empty list is not allowed when DISTINCT
       is used.

   Omitting the AS Key Word
       In the SQL standard, the optional key word AS can be omitted before an
       output column name whenever the new column name is a valid column name
       (that is, not the same as any reserved keyword).	 PostgreSQL is
       slightly more restrictive: AS is required if the new column name
       matches any keyword at all, reserved or not. Recommended practice is to
       use AS or double-quote output column names, to prevent any possible
       conflict against future keyword additions.

       In FROM items, both the standard and PostgreSQL allow AS to be omitted
       before an alias that is an unreserved keyword. But this is impractical
       for output column names, because of syntactic ambiguities.

   ONLY and Inheritance
       The SQL standard requires parentheses around the table name when
       writing ONLY, for example SELECT * FROM ONLY (tab1), ONLY (tab2) WHERE
       ....  PostgreSQL considers these parentheses to be optional.

       PostgreSQL allows a trailing * to be written to explicitly specify the
       non-ONLY behavior of including child tables. The standard does not
       allow this.

       (These points apply equally to all SQL commands supporting the ONLY
       option.)

   TABLESAMPLE Clause Restrictions
       The TABLESAMPLE clause is currently accepted only on regular tables and
       materialized views. According to the SQL standard it should be possible
       to apply it to any FROM item.

   Function Calls in FROM
       PostgreSQL allows a function call to be written directly as a member of
       the FROM list. In the SQL standard it would be necessary to wrap such a
       function call in a sub-SELECT; that is, the syntax FROM func(...) alias
       is approximately equivalent to FROM LATERAL (SELECT func(...)) alias.
       Note that LATERAL is considered to be implicit; this is because the
       standard requires LATERAL semantics for an UNNEST() item in FROM.
       PostgreSQL treats UNNEST() the same as other set-returning functions.

   Namespace Available to GROUP BY and ORDER BY
       In the SQL-92 standard, an ORDER BY clause can only use output column
       names or numbers, while a GROUP BY clause can only use expressions
       based on input column names.  PostgreSQL extends each of these clauses
       to allow the other choice as well (but it uses the standard's
       interpretation if there is ambiguity).  PostgreSQL also allows both
       clauses to specify arbitrary expressions. Note that names appearing in
       an expression will always be taken as input-column names, not as
       output-column names.

       SQL:1999 and later use a slightly different definition which is not
       entirely upward compatible with SQL-92. In most cases, however,
       PostgreSQL will interpret an ORDER BY or GROUP BY expression the same
       way SQL:1999 does.

   Functional Dependencies
       PostgreSQL recognizes functional dependency (allowing columns to be
       omitted from GROUP BY) only when a table's primary key is included in
       the GROUP BY list. The SQL standard specifies additional conditions
       that should be recognized.

   WINDOW Clause Restrictions
       The SQL standard provides additional options for the window
       frame_clause.  PostgreSQL currently supports only the options listed
       above.

   LIMIT and OFFSET
       The clauses LIMIT and OFFSET are PostgreSQL-specific syntax, also used
       by MySQL. The SQL:2008 standard has introduced the clauses OFFSET ...
       FETCH {FIRST|NEXT} ...  for the same functionality, as shown above in
       LIMIT Clause. This syntax is also used by IBM DB2. (Applications
       written for Oracle frequently use a workaround involving the
       automatically generated rownum column, which is not available in
       PostgreSQL, to implement the effects of these clauses.)

   FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, FOR KEY SHARE
       Although FOR UPDATE appears in the SQL standard, the standard allows it
       only as an option of DECLARE CURSOR.  PostgreSQL allows it in any
       SELECT query as well as in sub-SELECTs, but this is an extension. The
       FOR NO KEY UPDATE, FOR SHARE and FOR KEY SHARE variants, as well as the
       NOWAIT and SKIP LOCKED options, do not appear in the standard.

   Data-Modifying Statements in WITH
       PostgreSQL allows INSERT, UPDATE, and DELETE to be used as WITH
       queries. This is not found in the SQL standard.

   Nonstandard Clauses
       DISTINCT ON ( ... ) is an extension of the SQL standard.

       ROWS FROM( ... ) is an extension of the SQL standard.

PostgreSQL 10.1			     2017			     SELECT(7)
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