ALTER TABLE
SQL - Language Statements
ALTER TABLE
Modifies table properties
1999-07-20
ALTER TABLE table [ * ]
ADD [ COLUMN ] column type
ALTER TABLE table [ * ]
ALTER [ COLUMN ] column { SET DEFAULT value | DROP DEFAULT }
ALTER TABLE table [ * ]
RENAME [ COLUMN ] column TO newcolumn
ALTER TABLE table
RENAME TO newtable
ALTER TABLE table
ADD table constraint definition
1998-04-15
Inputs
table
The name of an existing table to alter.
column
Name of a new or existing column.
type
Type of the new column.
newcolumn
New name for an existing column.
newtable
New name for the table.
table constraint definition
New table constraint for the table
1998-04-15
Outputs
ALTER
Message returned from column or table renaming.
ERROR
Message returned if table or column is not available.
1998-04-15
Description
ALTER TABLE changes the definition of an existing table.
The ADD COLUMN form adds a new column to the table
using the same syntax as . The ALTER COLUMN form
allows you to set or remove the default for the column. Note that defaults
only apply to newly inserted rows.
The RENAME clause causes the name of a table or column
to change without changing any of the data contained in
the affected table. Thus, the table or column will
remain of the same type and size after this command is
executed.
The ADD table constraint definition clause
adds a new constraint to the table using the same syntax as .
You must own the table in order to change its schema.
1998-04-15
Notes
The keyword COLUMN is noise and can be omitted.
*
following a name of a table indicates that the statement
should be run over that table and all tables below it in the
inheritance hierarchy;
by default, the attribute will not be added to or renamed in any of the subclasses.
This should always be done when adding or modifying an attribute in a
superclass. If it is not, queries on the inheritance hierarchy
such as
SELECT NewColumn FROM SuperClass*
will not work because the subclasses will be missing an attribute
found in the superclass.
In the current implementation, default and constraint clauses for the
new column will be ignored. You can use the SET DEFAULT
form of ALTER TABLE to set the default later.
(You will also have to update the already existing rows to the
new default value, using .)
In the current implementation, only FOREIGN KEY constraints can
be added to a table. To create or remove a unique constraint, create
a unique index (see ). To add check constraints
you need to recreate and reload the table, using other
parameters to the command.
You must own the class in order to change its schema.
Renaming any part of the schema of a system
catalog is not permitted.
The PostgreSQL User's Guide has further
information on inheritance.
Refer to CREATE TABLE for a further description
of valid arguments.
Usage
To add a column of type VARCHAR to a table:
ALTER TABLE distributors ADD COLUMN address VARCHAR(30);
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 foreign key constraint to a table:
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses(address) MATCH FULL
Compatibility
1998-04-15
SQL92
The ADD COLUMN form is compliant with the exception that
it does not support defaults and constraints, as explained above.
The ALTER COLUMN form is in full compliance.
SQL92 specifies some additional capabilities for ALTER TABLE
statement which are not yet directly supported by Postgres:
ALTER TABLE table DROP CONSTRAINT constraint { RESTRICT | CASCADE }
Removes a table constraint (such as a check constraint,
unique constraint, or foreign key constraint). To
remove a unique constraint, drop a unique index,
To remove other kinds of constraints you need to recreate
and reload the table, using other parameters to the
command.
For example, to drop any constraints on a table distributors:
CREATE TABLE temp AS SELECT * FROM distributors;
DROP TABLE distributors;
CREATE TABLE distributors AS SELECT * FROM temp;
DROP TABLE temp;
ALTER TABLE table DROP [ COLUMN ] column { RESTRICT | CASCADE }
Removes a column from a table.
Currently, to remove an existing column the table must be
recreated and reloaded:
CREATE TABLE temp AS SELECT did, city FROM distributors;
DROP TABLE distributors;
CREATE TABLE distributors (
did DECIMAL(3) DEFAULT 1,
name VARCHAR(40) NOT NULL,
);
INSERT INTO distributors SELECT * FROM temp;
DROP TABLE temp;
The clauses to rename columns and tables are Postgres
extensions from SQL92.