ALTER TABLE adds or removes columns from a specified table. The default insertion value may also be changed. ALTER TABLE is not supported by transaction processing and cannot be undone.

ALTER TABLE table_name alter_table_action 

   alter_table_action ::=
       ADD [COLUMN] column_definition
      | DROP [COLUMN] column_name
      | ALTER [COLUMN] column_name {SET DEFAULT default_value | DROP DEFAULT}
      | ADD CONSTRAINT constraint_name UNIQUE(column_name)
      | DROP CONSTRAINT constraint_name 

   column_definition ::= column_name data_type [DEFAULT default_value]
   [{ NULL | NOT NULL | UNIQUE }] 

   data_type ::=
      VARCHAR(length) | DECIMAL(precision, scale)
      | NUMERIC(precision, scale) | SMALLINT | INTEGER
      | FLOAT(precision) | DOUBLE PRECISION | REAL | DATE