Basta usar LIKE, mas ao contrário do que você provavelmente costumava usar.
select query
from table1
where 'engine/mail/key' like concat(query,'%')
order by length(query) desc
limit 1
Resultados:
mysql> select * from query;
+-------------+
| query |
+-------------+
| engine |
| engine/pdf |
| engine/mail |
+-------------+
3 rows in set (0.00 sec)
mysql> select query from query
where 'engine/mail/key' like concat(query,'%')
order by length(query) desc
limit 1;
+-------------+
| query |
+-------------+
| engine/mail |
+-------------+
1 row in set (0.01 sec)
