{ SET UNUSED { COLUMN column
             | (column [, column ]...)
             }
  [ { CASCADE CONSTRAINTS | INVALIDATE }
      [ CASCADE CONSTRAINTS | INVALIDATE ]...
  ]
| DROP { COLUMN column
       | (column [, column ]...)
       }
  [ { CASCADE CONSTRAINTS | INVALIDATE }
      [ CASCADE CONSTRAINTS | INVALIDATE ]...
  ]
  [ CHECKPOINT integer ]
| DROP { UNUSED COLUMNS
       | COLUMNS CONTINUE
       }
  [ CHECKPOINT integer ]
}