MySQL 优化学习4 -- 子查询

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学生，课程，成绩三张表

mysql> desc test_user;
+----------+-------------+------+-----+---------+----------------+
| Field    | Type        | Null | Key | Default | Extra          |
+----------+-------------+------+-----+---------+----------------+
| id       | bigint(20)  | NO   | PRI | NULL    | auto_increment |
| username | varchar(50) | YES  | MUL | NULL    |                |
| email    | varchar(30) | YES  |     | NULL    |                |
| password | varchar(32) | YES  |     | NULL    |                |
| status   | tinyint(1)  | YES  |     | 0       |                |
| no       | int(11)     | YES  | MUL | 0       |                |
+----------+-------------+------+-----+---------+----------------+
6 rows in set (0.01 sec)

mysql> desc test_score;
+-----------+------------+------+-----+---------+----------------+
| Field     | Type       | Null | Key | Default | Extra          |
+-----------+------------+------+-----+---------+----------------+
| id        | bigint(20) | NO   | PRI | NULL    | auto_increment |
| user_id   | bigint(20) | YES  |     | NULL    |                |
| cource_id | bigint(20) | YES  |     | NULL    |                |
| score     | int(11)    | YES  |     | NULL    |                |
+-----------+------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

没有外键关联

出现如下的子查询,测试非常耗时

---

观察如下的几个查询，不同的语句查询时间差距比较大，join的效率比子查询，全表查效率要好不少。

select user_id, score from test_score where user_id in (select id from test_user where id < 100);

mysql> select user_id, score from test_score where user_id in (select id from test_user where id < 10);
+---------+-------+
| user_id | score |
+---------+-------+
|       1 |    46 |
|       2 |    78 |
|       3 |    51 |
|       4 |    21 |
|       5 |    54 |
|       6 |     6 |
|       7 |    70 |
|       8 |    30 |
|       9 |    41 |
+---------+-------+
9 rows in set (2.06 sec)

mysql>

mysql> select user_id, score from test_score join test_user using(id) where id < 10;
+---------+-------+
| user_id | score |
+---------+-------+
|       1 |    46 |
|       2 |    78 |
|       3 |    51 |
|       4 |    21 |
|       5 |    54 |
|       6 |     6 |
|       7 |    70 |
|       8 |    30 |
|       9 |    41 |
+---------+-------+
9 rows in set (0.00 sec)

mysql>

mysql> select user_id, score from test_score,test_user where test_score.user_id = test_user.id and test_user.id < 10;
+---------+-------+
| user_id | score |
+---------+-------+
|       1 |    46 |
|       2 |    78 |
|       3 |    51 |
|       4 |    21 |
|       5 |    54 |
|       6 |     6 |
|       7 |    70 |
|       8 |    30 |
|       9 |    41 |
+---------+-------+
9 rows in set (2.19 sec)

mysql>

• 参考
  sql的7中join: https://www.cnblogs.com/dinglinyong/p/6656315.html