Passing ENABLE AUTO_INCREMENT or DISABLE AUTO_INCREMENT enables or disables the 'Autoincrement' option for Longint type fields. Passing ENABLE LOG or DISABLE LOG enables or disables journaling for the table. Passing ENABLE REPLICATE or DISABLE REPLICATE enables or disables the mechanism allowing replication of the table (see the Replication via SQL section). Passing DROP CONSTRAINT sql_name removes the specified constraint from the table. Passing ADD foreign_key_definition adds a FOREIGN KEY to the table. Passing DROP PRIMARY KEY removes the PRIMARY KEY of the table. Passing ADD primary_key_definition adds a PRIMARY KEY to the table. Passing DROP sql_name removes the column named sql_name from the table. The PRIMARY KEY keyword is used to set the primary key when a column is added. Note: This command does not allow a field (column) of the Object type to be added. This option is useful when columns containing data have been deleted from the table (without the data being erased), to prevent existing data from being reassigned to the new column. The TRAILING keyword (which must be placed in front of column_definition if it is used) forces the column to be created after the last existing column of the table in the structure file. You can carry out one of the following actions: Passing ADD column_definition adds a column to the table. The ALTER TABLE command is used to modify an existing table ( sql_name).

Sql alter table add primary key