The standard requires the select lists of the selects for UNION
  to have the same number of columns

  Explanation:  The select lists of the two select expressions in
  this UNION have a different number of elements.  The ANSI and
  ISO SQL standards require that they have the same number of
  elements.

  User Action:  If it is important to have only standard features
  in your program, you should change your selects to have the same
  number of items in the select list.