ALTER TABLE(l)SQL - Language Statements (2002-11-22ALTER TABLE(l)
NAME
ALTER TABLE - change the definition of a table
SYNOPSIS
ALTER TABLE [ ONLY ] table [ * ]
ADD [ COLUMN ] column type [ column_constraint [ ... ] ]
ALTER TABLE [ ONLY ] table [ * ]
DROP [ COLUMN ] column [ RESTRICT | CASCADE ]
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column { SET DEFAULT value | DROP DEFAULT }
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column { SET | DROP } NOT NULL
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column SET STATISTICS integer
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ALTER TABLE [ ONLY ] table [ * ]
RENAME [ COLUMN ] column TO new_column
ALTER TABLE table
RENAME TO new_table
ALTER TABLE [ ONLY ] table [ * ]
ADD table_constraint
ALTER TABLE [ ONLY ] table [ * ]
DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]
ALTER TABLE table
OWNER TO new_owner
INPUTS
table
The name (possibly schema-qualified) of an existing
table to alter. If ONLY is specified, only that table
is altered. If ONLY is not specified, the table and all
its descendant tables (if any) are updated. * can be
appended to the table name to indicate that descendant
tables are to be scanned, but in the current version,
this is the default behavior. (In releases before 7.1,
ONLY was the default behavior.) The default can be
altered by changing the SQL_INHERITANCE configuration
option.
column
Name of a new or existing column.
type Type of the new column.
new_column
New name for an existing column.
new_table
New name for the table.
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table_constraint
New table constraint for the table.
constraint_name
Name of an existing constraint to drop.
new_owner
The user name of the new owner of the table.
CASCADE
Automatically drop objects that depend on the dropped
column or constraint (for example, views referencing
the column).
RESTRICT
Refuse to drop the column or constraint if there are
any dependent objects. This is the default behavior.
OUTPUTS
ALTER TABLE
Message returned from column or table renaming.
ERROR
Message returned if table or column is not available.
DESCRIPTION
ALTER TABLE changes the definition of an existing table.
There are several sub-forms:
ADD COLUMN
This form adds a new column to the table using the same
syntax as CREATE TABLE [create_table(l)].
DROP COLUMN
This form drops a column from a table. Note that
indexes and table constraints involving the column will
be automatically dropped as well. You will need to say
CASCADE if anything outside the table depends on the
column --- for example, foreign key references, views,
etc.
SET/DROP DEFAULT
These forms set or remove the default value for a
column. Note that defaults only apply to subsequent
INSERT commands; they do not cause rows already in the
table to change. Defaults may also be created for
views, in which case they are inserted into INSERT
statements on the view before the view's ON INSERT rule
is applied.
SET/DROP NOT NULL
These forms change whether a column is marked to allow
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NULL values or to reject NULL values. You may only SET
NOT NULL when the table contains no null values in the
column.
SET STATISTICS
This form sets the per-column statistics-gathering
target for subsequent ANALYZE [analyze(l)] operations.
The target can be set in the range 0 to 1000;
alternatively, set it to -1 to revert to using the
system default statistics target.
SET STORAGE
This form sets the storage mode for a column. This
controls whether this column is held inline or in a
supplementary table, and whether the data should be
compressed or not. PLAIN must be used for fixed-length
values such as INTEGER and is inline, uncompressed.
MAIN is for inline, compressible data. EXTERNAL is for
external, uncompressed data and EXTENDED is for
external, compressed data. EXTENDED is the default for
all data types that support it. The use of EXTERNAL
will make substring operations on a TEXT column faster,
at the penalty of increased storage space.
RENAME
The RENAME forms change the name of a table (or an
index, sequence, or view) or the name of an individual
column in a table. There is no effect on the stored
data.
ADD table_constraint
This form adds a new constraint to a table using the
same syntax as CREATE TABLE [create_table(l)].
DROP CONSTRAINT
This form drops constraints on a table. Currently,
constraints on tables are not required to have unique
names, so there may be more than one constraint
matching the specified name. All such constraints will
be dropped.
OWNER
This form changes the owner of the table, index,
sequence or view to the specified user.
You must own the table to use ALTER TABLE; except for ALTER
TABLE OWNER, which may only be executed by a superuser.
NOTES
The keyword COLUMN is noise and can be omitted.
In the current implementation of ADD COLUMN, default and NOT
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NULL clauses for the new column are not supported. The new
column always comes into being with all values NULL. You
can use the SET DEFAULT form of ALTER TABLE to set the
default afterwards. (You may also want to update the
already existing rows to the new default value, using UPDATE
[update(l)].) If you want to mark the column non-null, use
the SET NOT NULL form after you've entered non-null values
for the column in all rows.
The DROP COLUMN command does not physically remove the
column, but simply makes it invisible to SQL operations.
Subsequent inserts and updates of the table will store a
NULL for the column. Thus, dropping a column is quick but
it will not immediately reduce the on-disk size of your
table, as the space occupied by the dropped column is not
reclaimed. The space will be reclaimed over time as existing
rows are updated. To reclaim the space at once, do a dummy
UPDATE of all rows and then vacuum, as in:
UPDATE table SET col = col;
VACUUM FULL table;
If a table has any descendant tables, it is not permitted to
ADD or RENAME a column in the parent table without doing the
same to the descendants --- that is, ALTER TABLE ONLY will
be rejected. This ensures that the descendants always have
columns matching the parent.
A recursive DROP COLUMN operation will remove a descendant
table's column only if the descendant does not inherit that
column from any other parents and never had an independent
definition of the column. A nonrecursive DROP COLUMN (i.e.,
ALTER TABLE ONLY ... DROP COLUMN) never removes any
descendant columns, but instead marks them as independently
defined rather than inherited.
Changing any part of the schema of a system catalog is not
permitted.
Refer to CREATE TABLE for a further description of valid
arguments. The PostgreSQL User's Guide has further
information on inheritance.
USAGE
To add a column of type varchar to a table:
ALTER TABLE distributors ADD COLUMN address VARCHAR(30);
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To drop a column from a table:
ALTER TABLE distributors DROP COLUMN address RESTRICT;
To rename an existing column:
ALTER TABLE distributors RENAME COLUMN address TO city;
To rename an existing table:
ALTER TABLE distributors RENAME TO suppliers;
To add a NOT NULL constraint to a column:
ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;
To remove a NOT NULL constraint from a column:
ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;
To add a check constraint to a table:
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
To remove a check constraint from a table and all its
children:
ALTER TABLE distributors DROP CONSTRAINT zipchk;
To add a foreign key constraint to a table:
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses(address) MATCH FULL;
To add a (multicolumn) unique constraint to a table:
ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);
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To add an automatically named primary key constraint to a
table, noting that a table can only ever have one primary
key:
ALTER TABLE distributors ADD PRIMARY KEY (dist_id);
COMPATIBILITY
SQL92
The ADD COLUMN form is compliant with the exception that it
does not support defaults and NOT NULL constraints, as
explained above. The ALTER COLUMN form is in full
compliance.
The clauses to rename tables, columns, indexes, and
sequences are PostgreSQL extensions from SQL92.
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