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，先执行left join，然后再根据where条件进行过滤

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