SQL多个JOIN执行顺序

• 多个JOIN从左到右执行
• select * from a left join b on a.id=b.id left join c on b.id=c.id 先a和b交的结果集t，t再和c相交

mysql> select * from a;
+------+------+
| id   | name |
+------+------+
|    1 | a1   |
|    2 | a2   |
|    3 | a3   |
|    4 | a4   |
+------+------+
4 rows in set (0.00 sec)

mysql> select * from b;
+------+------+
| id   | name |
+------+------+
|    1 | b1   |
|    2 | b2   |
|    5 | b5   |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from c;
+------+------+
| id   | name |
+------+------+
|    1 | c1   |
|    4 | c4   |
|    5 | c5   |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from a left join b on a.id=b.id;
+------+------+------+------+
| id   | name | id   | name |
+------+------+------+------+
|    1 | a1   |    1 | b1   |
|    2 | a2   |    2 | b2   |
|    3 | a3   | NULL | NULL |
|    4 | a4   | NULL | NULL |
+------+------+------+------+
4 rows in set (0.00 sec)

mysql> select * from a left join b on a.id=b.id left join c on b.id=c.id;
+------+------+------+------+------+------+
| id   | name | id   | name | id   | name |
+------+------+------+------+------+------+
|    1 | a1   |    1 | b1   |    1 | c1   |
|    2 | a2   |    2 | b2   | NULL | NULL |
|    3 | a3   | NULL | NULL | NULL | NULL |
|    4 | a4   | NULL | NULL | NULL | NULL |
+------+------+------+------+------+------+
4 rows in set (0.00 sec)

--- Block Nested Loop 是驱动表（左边表）逐条和被驱动表（右边表）比较
mysql> explain select * from a left join b on a.id=b.id left join c on b.id=c.id;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------
---------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra
                                             |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------
---------------------------------------------+
|  1 | SIMPLE      | a     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL
                                             |
|  1 | SIMPLE      | b     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using
where; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | c     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using
where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------
---------------------------------------------+
3 rows in set, 1 warning (0.00 sec)

转载于:https://my.oschina.net/u/3960369/blog/2988890