sql中的left join以及on、where关键字的区别

创建两张表并插入一些数据

LEFT JOIN 关键字会从左表 (table_www.bifa9999com,name1) 那里返回所有的行,即使在右表
(table_name2) 中没有匹配的行。

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);

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

查看表中的数据

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

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>

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);

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

查看表中的数据

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;
+----------+------------+-------------+
| 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

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

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;
+----------+------------+-------------+----------+--------+-------+
| 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 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>
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。

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>
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>

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。

/**********************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

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>

不管on后面有哪些条件,left join都要返回左表中的所有行!

/**********************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:

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> 

(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

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

不管on后面有哪些条件,left join都要返回左表中的所有行!

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

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脚本之家网站的支持!

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