{ 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 ] }