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UPDATE()			 SQL Commands			      UPDATE()

NAME
       UPDATE - update rows of a table

SYNOPSIS
       UPDATE [ ONLY ] table [ [ AS ] alias ]
	   SET { column = { expression | DEFAULT } |
		 ( column [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...]
	   [ FROM fromlist ]
	   [ WHERE condition ]
	   [ RETURNING * | output_expression [ AS output_name ] [, ...] ]

DESCRIPTION
       UPDATE  changes	the  values  of the specified columns in all rows that
       satisfy the condition. Only the columns to be  modified	need  be  men‐
       tioned  in the SET clause; columns not explicitly modified retain their
       previous values.

       By default, UPDATE will update rows in the specified table and all  its
       subtables. If you wish to only update the specific table mentioned, you
       must use the ONLY clause.

       There are two ways to modify a table  using  information	 contained  in
       other  tables  in  the database: using sub-selects, or specifying addi‐
       tional tables in the FROM clause. Which technique is  more  appropriate
       depends on the specific circumstances.

       The  optional  RETURNING	 clause	 causes	 UPDATE	 to compute and return
       value(s) based on each row actually updated.  Any expression using  the
       table's	columns, and/or columns of other tables mentioned in FROM, can
       be computed.  The new (post-update) values of the table's  columns  are
       used.   The  syntax  of	the RETURNING list is identical to that of the
       output list of SELECT.

       You must have the UPDATE privilege on the table to update it,  as  well
       as  the	SELECT	privilege  to  any  table whose values are read in the
       expressions or condition.

PARAMETERS
       table  The name (optionally schema-qualified) of the table to update.

       alias  A substitute name for the target table. When an  alias  is  pro‐
	      vided,  it  completely  hides  the actual name of the table. For
	      example, given UPDATE foo AS f,  the  remainder  of  the	UPDATE
	      statement must refer to this table as f not foo.

       column The name of a column in table.  The column name can be qualified
	      with a subfield name or  array  subscript,  if  needed.  Do  not
	      include the table's name in the specification of a target column
	      — for example, UPDATE tab SET tab.col = 1 is invalid.

       expression
	      An expression to assign to the column. The  expression  may  use
	      the old values of this and other columns in the table.

       DEFAULT
	      Set  the	column	to its default value (which will be NULL if no
	      specific default expression has been assigned to it).

       fromlist
	      A list of table expressions, allowing columns from other	tables
	      to  appear  in  the  WHERE condition and the update expressions.
	      This is similar to the list of tables that can be	 specified  in
	      the FROM Clause [select(5)] of a SELECT statement. Note that the
	      target table must not appear in the fromlist, unless you	intend
	      a	 self-join  (in which case it must appear with an alias in the
	      fromlist).

       condition
	      An expression that returns a value of type boolean.   Only  rows
	      for which this expression returns true will be updated.

       output_expression
	      An  expression to be computed and returned by the UPDATE command
	      after each row is updated. The expression	 may  use  any	column
	      names  of	 the  table  or	 table(s)  listed in FROM.  Write * to
	      return all columns.

       output_name
	      A name to use for a returned column.

OUTPUTS
       On successful completion, an UPDATE command returns a  command  tag  of
       the form

       UPDATE count

       The count is the number of rows updated. If count is 0, no rows matched
       the condition (this is not considered an error).

       If the UPDATE command contains a RETURNING clause, the result  will  be
       similar to that of a SELECT statement containing the columns and values
       defined in the RETURNING list, computed over the row(s) updated by  the
       command.

NOTES
       When  a	FROM  clause  is present, what essentially happens is that the
       target table is joined to the tables mentioned  in  the	fromlist,  and
       each output row of the join represents an update operation for the tar‐
       get table. When using FROM you should ensure that the join produces  at
       most one output row for each row to be modified. In other words, a tar‐
       get row shouldn't join to more than one row from the other table(s). If
       it does, then only one of the join rows will be used to update the tar‐
       get row, but which one will be used is not readily predictable.

       Because of this indeterminacy, referencing  other  tables  only	within
       sub-selects is safer, though often harder to read and slower than using
       a join.

EXAMPLES
       Change the word Drama to Dramatic in  the  column  kind	of  the	 table
       films:

       UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama';

       Adjust temperature entries and reset precipitation to its default value
       in one row of the table weather:

       UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
	 WHERE city = 'San Francisco' AND date = '2003-07-03';

       Perform the same operation and return the updated entries:

       UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
	 WHERE city = 'San Francisco' AND date = '2003-07-03'
	 RETURNING temp_lo, temp_hi, prcp;

       Use the alternative column-list syntax to do the same update:

       UPDATE weather SET (temp_lo, temp_hi, prcp) = (temp_lo+1, temp_lo+15, DEFAULT)
	 WHERE city = 'San Francisco' AND date = '2003-07-03';

       Increment the sales count of the salesperson who	 manages  the  account
       for Acme Corporation, using the FROM clause syntax:

       UPDATE employees SET sales_count = sales_count + 1 FROM accounts
	 WHERE accounts.name = 'Acme Corporation'
	 AND employees.id = accounts.sales_person;

       Perform the same operation, using a sub-select in the WHERE clause:

       UPDATE employees SET sales_count = sales_count + 1 WHERE id =
	 (SELECT sales_person FROM accounts WHERE name = 'Acme Corporation');

       Attempt to insert a new stock item along with the quantity of stock. If
       the item already exists, instead update the stock count of the existing
       item.  To  do  this  without  failing the entire transaction, use save‐
       points.

       BEGIN;
       -- other operations
       SAVEPOINT sp1;
       INSERT INTO wines VALUES('Chateau Lafite 2003', '24');
       -- Assume the above fails because of a unique key violation,
       -- so now we issue these commands:
       ROLLBACK TO sp1;
       UPDATE wines SET stock = stock + 24 WHERE winename = 'Chateau Lafite 2003';
       -- continue with other operations, and eventually
       COMMIT;

COMPATIBILITY
       This command conforms to the SQL standard, except  that	the  FROM  and
       RETURNING clauses are PostgreSQL extensions.

       According  to  the standard, the column-list syntax should allow a list
       of columns to be assigned from a single row-valued expression, such  as
       a sub-select:

       UPDATE accounts SET (contact_last_name, contact_first_name) =
	   (SELECT last_name, first_name FROM salesmen
	    WHERE salesmen.id = accounts.sales_id);

       This  is not currently implemented — the source must be a list of inde‐
       pendent expressions.

       Some other database systems offer a FROM option in which the target ta‐
       ble  is supposed to be listed again within FROM.	 That is not how Post‐
       greSQL interprets FROM. Be careful when porting applications  that  use
       this extension.

SQL - Language Statements	  2008-06-08			      UPDATE()
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