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
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SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2
ON table_name1.column_name=table_name2.column_name
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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
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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);
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查看表中的数据
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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>
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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
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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)
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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！
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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>
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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。
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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>
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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。
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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>
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/**********************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！
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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>
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/**********************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服务器之家网站的支持！
原文链接：https://www.cnblogs.com/abclife/archive/2018/08/30/9559658.html