A common problem using the UNION command in MySQL is when you want to order data sets differently. If you do it incorrectly, you will see the error message:
Incorrect usage of UNION and ORDER BY
To solve the problem, wrap each SELECT statement in parenthesis as follows:
(SELECT col1,col2,col3 FROM table1 ORDER BY col2)
(SELECT cola,colb,colc FROM table2 ORDER BY cola)
Nice and simple; all you have to do is remember the brackets!