{ 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]
}

For information on the symbols and conventions of Backus-Naur Form (BNF) syntax, see Appendix A.