LEFT JOIN 关键字会从左表 (table_name1) 那里返回所有的行，即使在右表 (table_name2) 中没有匹配的行。

LEFT JOIN 关键字语法

SELECT column_name(s)FROM table_name1LEFT JOIN table_name2 ON table_name1.column_name=table_name2.column_name

注释：在某些数据库中， LEFT JOIN 称为 LEFT OUTER JOIN。

创建两张表并插入一些数据

create table class( class_id int, class_name varchar(20), class_grade char(1));insert into class values (1,'语文','A');insert into class values (2,'数学','B');insert into class values (3,'英语','C');create table score( class_id int, stu_id varchar(20), Score int);insert into score values (1,'A001',91);insert into score values (2,'A001',95);insert into score values (1,'A002',82);insert into score values (2,'A002',87);insert into score values (3,'B003',65);

查看表中的数据

mysql> select * from class;+----------+------------+-------------+| class_id | class_name | class_grade |+----------+------------+-------------+| 1 | 语文 | A || 2 | 数学 | B || 3 | 英语 | C |+----------+------------+-------------+3 rows in set (0.00 sec)mysql> select * from score;+----------+--------+-------+| class_id | stu_id | Score |+----------+--------+-------+| 1 | A001 | 91 || 2 | A001 | 95 || 1 | A002 | 82 || 2 | A002 | 87 || 3 | B003 | 65 |+----------+--------+-------+5 rows in set (0.00 sec)mysql>

比较下面几组查询结果

--如果你对left join足够熟悉的话，先不要看结果，是否可以直接说出下面查询的结果

mysql> select * from class A left join score B on A.class_id=B.class_id;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | 1 | A001 | 91 || 2 | 数学 | B | 2 | A001 | 95 || 1 | 语文 | A | 1 | A002 | 82 || 2 | 数学 | B | 2 | A002 | 87 || 3 | 英语 | C | 3 | B003 | 65 |+----------+------------+-------------+----------+--------+-------+5 rows in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and 1=1;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | 1 | A001 | 91 || 2 | 数学 | B | 2 | A001 | 95 || 1 | 语文 | A | 1 | A002 | 82 || 2 | 数学 | B | 2 | A002 | 87 || 3 | 英语 | C | 3 | B003 | 65 |+----------+------------+-------------+----------+--------+-------+5 rows in set (0.01 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and 1=0;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.00 sec)mysql> select * from class A left join score B on 1=0;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.00 sec)

mysql> left join的最重要特点是：不管on后面是什么条件，都会返回左表中的所有行！

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='语文';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | 1 | A001 | 91 || 1 | 语文 | A | 1 | A002 | 82 || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+4 rows in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='数学';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 2 | 数学 | B | 2 | A001 | 95 || 2 | 数学 | B | 2 | A002 | 87 || 1 | 语文 | A | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+4 rows in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='英语';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 3 | 英语 | C | 3 | B003 | 65 || 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.01 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='体育';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.00 sec)mysql>

如果on后面的条件是左表中的列(and leftTable.colName='***'),左表中满足条件的行和右表中的行进行匹配(根据on leftTable.id=rightTable.id)；左表中不满足条件的行，直接输出，其对应的右表中的列都是null。

mysql> select * from class A left join score B on A.class_id=B.class_id and B.Score=90;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.01 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and B.Score=65;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 3 | 英语 | C | 3 | B003 | 65 || 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.01 sec)mysql>

如果on后面的条件是右表中的列(and rightTable.colName='***'),首先会根据(and rightTable.colName='***')过滤掉右表中不满足条件的行；然后，左表中的行根据(on leftTable.id=rightTable.id)和右表中满足条件的行进行匹配。

mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='语文' and B.Score=90;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='语文' and B.Score=91;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | 1 | A001 | 91 || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.01 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='体育' and B.Score=90;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id and A.class_name='体育' and B.Score=82;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | NULL | NULL | NULL || 2 | 数学 | B | NULL | NULL | NULL || 3 | 英语 | C | NULL | NULL | NULL |+----------+------------+-------------+----------+--------+-------+3 rows in set (0.00 sec)mysql>

/**********************过滤条件在on中时**********************/

总结一下，如果 left join on leftTable.id=rightTable.id 后还有其他条件：

(1)and leftTable.colName='***'，过滤左表，但是左表不满足条件的行直接输出，并将右表对应部分置为null
(2)and rightTable.colName='***'，过滤右表，对左表没有影响
(3)and leftTable.colName='***' and rightTable.colName='***'，就是上面(1)和(2)一起发挥作用

不管on后面有哪些条件，left join都要返回左表中的所有行！

mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='语文';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | 1 | A001 | 91 || 1 | 语文 | A | 1 | A002 | 82 |+----------+------------+-------------+----------+--------+-------+2 rows in set (0.01 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='数学';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 2 | 数学 | B | 2 | A001 | 95 || 2 | 数学 | B | 2 | A002 | 87 |+----------+------------+-------------+----------+--------+-------+2 rows in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='英语';+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 3 | 英语 | C | 3 | B003 | 65 |+----------+------------+-------------+----------+--------+-------+1 row in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='体育';Empty set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where B.Score=90;Empty set (0.01 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where B.Score=91;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | 1 | A001 | 91 |+----------+------------+-------------+----------+--------+-------+1 row in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='语文' and B.Score=90;Empty set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='语文' and B.Score=91;+----------+------------+-------------+----------+--------+-------+| class_id | class_name | class_grade | class_id | stu_id | Score |+----------+------------+-------------+----------+--------+-------+| 1 | 语文 | A | 1 | A001 | 91 |+----------+------------+-------------+----------+--------+-------+1 row in set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='体育' and B.Score=90;Empty set (0.00 sec)mysql> select * from class A left join score B on A.class_id=B.class_id where A.class_name='体育' and B.Score=91;Empty set (0.00 sec)mysql>

/**********************过滤条件在where中时**********************/

过滤条件写在where中时，先根据where条件对表进行过滤，然后再执行left join

总结

以上所述是小编给大家介绍的sql中的left join及on、where关键字的区别详解，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对网站的支持！