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_name1 left 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www.887551.com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