子查询与连表查询

标签:数据库

子查询

mysql> explain select e.empno,e.ename,(select dname from dept d where e.deptno = d.deptno) as dname from emp e where e.deptno = 1;
+----+--------------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
| id | select_type        | table | partitions | type   | possible_keys | key     | key_len | ref            | rows | filtered | Extra       |
+----+--------------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
|  1 | PRIMARY            | e     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL           |   47 |    10.00 | Using where |
|  2 | DEPENDENT SUBQUERY | d     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | scott.e.deptno |    1 |   100.00 | NULL        |
+----+--------------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
2 rows in set (0.07 sec)

连表查询

mysql> explain select e.empno,e.ename,d.dname from emp e join dept d on e.deptno = d.deptno where  e.deptno = 1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | d     | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  1 | SIMPLE      | e     | NULL       | ALL   | NULL          | NULL    | NULL    | NULL  |   47 |    10.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
2 rows in set (0.22 sec)

可以看出连表查询被优化器优化成了dept表关联empt表

子查询

mysql> explain select e.empno,e.ename,(select dname from dept d where e.deptno = d.deptno) as dname from emp e where e.job = '五散人';
+----+--------------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
| id | select_type        | table | partitions | type   | possible_keys | key     | key_len | ref            | rows | filtered | Extra       |
+----+--------------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
|  1 | PRIMARY            | e     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL           |   47 |    10.00 | Using where |
|  2 | DEPENDENT SUBQUERY | d     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | scott.e.deptno |    1 |   100.00 | NULL        |
+----+--------------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
2 rows in set (0.07 sec)

连表查询

mysql> explain select e.empno,e.ename,d.dname from emp e join dept d on e.deptno = d.deptno where e.job = '五散人';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                              |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | d     | NULL       | ALL  | PRIMARY       | NULL | NULL    | NULL |    4 |   100.00 | NULL                                               |
|  1 | SIMPLE      | e     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   47 |     2.13 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set (0.07 sec)