I’ve you’re just starting to play with a Union, then you might have gotten the error message:
#1221 - Incorrect usage of UNION and ORDER BY
You might be thinking: Whoa, hang on there! Do you mean MySQL can't order a Union?! Oh, but it can! First little issue I had was in realizing that you can put parenthesis around the queries. So phrase your query thusly:
"(SELECT ...) UNION (SELECT ...) ORDER BY ..."
Just be sure that your Order By includes only the column name. Table.Column is going to get you another error!