The unioned column type may be the smallest type that is guaranteed to be large enough to hold values from all tables.
To use ORDER BY and LIMIT clauses to the unioned table, surround each SELECT with parentheses and then add ORDER BY or LIMIT after the last parenthesis. Collumns named in such an ORDER BY should refer to columns in the first SELECT of the statement.

   (SELECT colA, colB FROM tableA)
   (SELECT colC, colD FROM tableB)
   ORDER BY colA LIMIT 10;