EXPLAIN SELECT * FROM staff WHERE department = "Cooking" AND address="Dallas";

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | staff | ALL  | NULL          | NULL | NULL    | NULL |    7 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

CREATE INDEX address_idx ON staff(address);
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

EXPLAIN SELECT * FROM staff WHERE department = "Cooking" AND address="Dallas";
+------+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+
| id   | select_type | table | type | possible_keys | key         | key_len | ref   | rows | Extra                              |
+------+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+
|    1 | SIMPLE      | staff | ref  | address_idx   | address_idx | 130     | const |    2 | Using index condition; Using where |
+------+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+