Use uma declaração preparada:
mysql> SET @sql = CONCAT("SELECT ", (SELECT "NOW()"));
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT @sql;
+--------------+
| @sql |
+--------------+
| SELECT NOW() |
+--------------+
1 row in set (0.00 sec)
mysql> PREPARE stmt FROM @sql;
Query OK, 0 rows affected (0.00 sec)
Statement prepared
mysql> EXECUTE stmt;
+---------------------+
| NOW() |
+---------------------+
| 2009-04-06 23:08:31 |
+---------------------+
1 row in set (0.00 sec)
