mysql> EXPLAIN SELECT DISTINCT c.sic1 FROM company_sic c JOIN sic_lookup l ON(c.sic1 = l.sic1);
+-------+-------+---------------+------+---------+--------+--------+------------------------------------+
| table | type | possible_keys | key | key_len | ref | rows | Extra |
+-------+-------+---------------+------+---------+--------+--------+------------------------------------+
| c | index | SIC | SIC | 6 | NULL | 283437 | Using index; Using temporary |
| l | ref | SIC | SIC | 2 | c.sic1 | 137 | Using where; Using index; Distinct |
+-------+-------+---------------+------+---------+--------+--------+------------------------------------+
2 rows in set (0.13 sec)