в MySQL использовать UNION после ORDER нельзя:
SELECT 1 ORDER BY 1 LIMIT 1 UNION SELECT 1
->
Incorrect usage of UNION and ORDER BY
но просто с LIMIT можно:
SELECT 1 LIMIT 1 UNION SELECT 1;
->
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)