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

  > 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

  > 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)

> 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沃谷博客网站的支持!

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