这可以用以下解释
mysql> select * from table1 ;+------+------+| id | val |+------+------+| 1 | 10 || 2 | 30 || 3 | 40 |+------+------+3 rows in set (0.00 sec)mysql> select * from table2 ;+------+------+| id | t1id |+------+------+| 1 | 1 || 2 | 2 |+------+------+2 rows in set (0.00 sec)
这里
table1.id <-> table2.t1id
现在，当我们
leftjoin使用连接键进行操作时，如果左边的表是table1，那么它将从table1获取所有数据，并且在table2的不匹配记录中将其设置为null
mysql> select t1.* , t2.t1id from table1 t1 left join table2 t2 on t2.t1id = t1.id ;+------+------+------+| id | val | t1id |+------+------+------+| 1 | 10 | 1 || 2 | 30 | 2 || 3 | 40 | NULL |+------+------+------+3 rows in set (0.00 sec)
看到table1.id = 3在table2中没有值，因此将其设置为null当您应用where条件时，它将进行进一步的过滤
mysql> select t1.* , t2.t1id from table1 t1 left join table2 t2 on t2.t1id = t1.id where t2.t1id is null;+------+------+------+| id | val | t1id |+------+------+------+| 3 | 40 | NULL |+------+------+------+1 row in set (0.00 sec)
