The standard requires columns merged by UNION be identical

  Explanation:  The ANSI and ISO SQL standards require that
  columns which are being merged into a single result column by a
  UNION clause must have identical definitions; that is, they must
  have the same data type, length, scale, and so on.  The column
  pointed to is not identical to the column with which it is being
  merged.

  User Action:  If it is important to have only standard features
  in your program, you should change the data types of these
  columns to be identical, or remove the UNION from your program.