{ 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]
}