PostgreSQL 7.2 Documentation
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# ALTER TABLE

## Name

ALTER TABLE -- change the definition of a table

## Synopsis

    ALTER TABLE [ ONLY ] table [ * ]
        ADD [ COLUMN ] column type [ column_constraint [ ... ] ]
    ALTER TABLE [ ONLY ] table [ * ]
        ALTER [ COLUMN ] column { SET DEFAULT value | DROP DEFAULT }
    ALTER TABLE [ ONLY ] table [ * ]
        ALTER [ COLUMN ] column SET STATISTICS integer
    ALTER TABLE [ ONLY ] table [ * ]
        RENAME [ COLUMN ] column TO newcolumn
    ALTER TABLE table
        RENAME TO newtable
    ALTER TABLE table
        ADD table constraint definition
    ALTER TABLE [ ONLY ] table
    	DROP CONSTRAINT constraint { RESTRICT | CASCADE }
    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 SET/DROP DEFAULT` forms allow you to set or remove the default for the column. Note that defaults only apply to subsequent INSERT commands; they do not cause rows already in the table to change. The `ALTER COLUMN SET STATISTICS` form allows you to set the statistics-gathering target for subsequent ANALYZE operations. The `RENAME` clause causes the name of a table, column, index, or sequence to change without changing any of the data. The data will remain of the same type and size after the command is executed. The ADD `table constraint definition` clause adds a new constraint to the table using the same syntax as CREATE TABLE. The DROP CONSTRAINT `constraint` clause drops all constraints on the table (and its children) that match `constraint`. The OWNER clause changes 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 of `ADD COLUMN`, default and NOT NULL clauses for the new column are not supported. You can use the `SET DEFAULT` form of ALTER TABLE to set the default later. (You may also want to update the already existing rows to the new default value, using UPDATE.)

In DROP CONSTRAINT, the RESTRICT keyword is required, although dependencies are not yet checked. The CASCADE option is unsupported. Currently DROP CONSTRAINT drops only CHECK constraints. To remove a PRIMARY or UNIQUE constraint, drop the relevant index using the DROP INDEX command. To remove FOREIGN KEY constraints you need to recreate and reload the table, using other parameters to the CREATE TABLE command.

For example, to drop all 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;

You must own the table in order to change it. Changing 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 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 RESTRICT;

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);

## 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.

SQL92 specifies some additional capabilities for ALTER TABLE statement which are not yet directly supported by PostgreSQL:

    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 tables, columns, indexes, and sequences are PostgreSQL extensions from SQL92.

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