复制mysql> explain SELECT * FROM dept_emp WHERE emp_no IN (SELECT emp_no FROM dept_emp GROUPBY emp_no HAVINGCOUNT(emp_no)>1);  +----+-------------+----------+------------+-------+-----------------+---------+---------+------+--------+----------+-------------+ | id | select_type | table    | partitions | type  | possible_keys   | key     | key_len | ref  | rows   | filtered | Extra       |  +----+-------------+----------+------------+-------+-----------------+---------+---------+------+--------+----------+-------------+ |  1 | PRIMARY     | dept_emp | NULL       | ALL   | NULL            | NULL    | NULL    | NULL | 331143 |   100.00 | Using where |  |  2 | SUBQUERY    | dept_emp | NULL       | index | PRIMARY,看懂dept_no | PRIMARY | 16      | NULL | 331143 |   100.00 | Using index |  +----+-------------+----------+------------+-------+-----------------+---------+---------+------+--------+----------+-------------+ 1.2.3.4.5.6.7.