修改、删除和新增约束

constraint_clauses:
{ ADD { { out_of_line_constraint }...
      | out_of_line_REF_constraint
      }
| MODIFY { CONSTRAINT constraint_name
         | PRIMARY KEY
         | UNIQUE (column [, column ]...)
         } constraint_state [ CASCADE ]
| RENAME CONSTRAINT old_name TO new_name
| { drop_constraint_clause }...
}

out_of_line_constraint:
[ CONSTRAINT constraint_name ]
{ UNIQUE (column [, column ]...)
| PRIMARY KEY (column [, column ]...)
| FOREIGN KEY (column [, column ]...) references_clause
| CHECK (condition)
} [ constraint_state ]

out_of_line_REF_constraint:
{ SCOPE FOR ({ ref_col | ref_attr })
    IS [ schema. ] scope_table
| REF ({ ref_col | ref_attr }) WITH ROWID
| [ CONSTRAINT constraint_name ] FOREIGN KEY
    ( { ref_col [, ref_col ] | ref_attr [, ref_attr ] } ) references_clause
    [ constraint_state ]
}

drop_constraint_clause:
(
  DROP ( ( PRIMARY KEY
         | UNIQUE "(" (column)... ")"  
         | CONSTRAINT constraint_name ) 
   [ CASCADE ] [( KEEP | DROP ) INDEX ]
    ) 
    [ ONLINE ]
)

ALTER TABLE T DROP CONSTRAINT CST;