mysql> select count(*) from users U join challenges C where C.level=10;
+----------+
| count(*) |
+----------+
|     7296 | 
+----------+
1 row in set (0.01 sec)

mysql> select count(*) from users U inner join challenges C on U.challenge_id = C.id where C.level=10;
+----------+
| count(*) |
+----------+
|       61 | 
+----------+
1 row in set (0.01 sec)