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ALTER TABLE

Name

ALTER TABLE  --  Modifies table properties

Synopsis

ALTER TABLE [ ONLY ] table [ * ]
    ADD [ COLUMN ] column type
ALTER TABLE [ ONLY ] 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
ALTER TABLE table
        OWNER TO new owner 
  

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

New user

The user name of the new owner of the table.

Outputs

ALTER

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. The ADD COLUMN form adds a new column to the table using the same syntax as CREATE TABLE. 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 CREATE TABLE. The OWNER clause chnages the owner of the table to the user new user.

You must own the table in order to change its schema.

Notes

The keyword COLUMN is noise and can be omitted.

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 UPDATE.)

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 CREATE INDEX). To add check constraints you need to recreate and reload the table, using other parameters to the CREATE TABLE command.

You must own the table in order to change it. 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

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 CREATE TABLE 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.

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