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

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

sql中的left join以及on、where关键字的区别的更多相关文章

Linq To Sql中实现Left Join与Inner Join使用Linq语法与lambda表达式
当前有两个表,sgroup与sgroupuser,两者通过gKey关联,而sgroup表记录的是组,而sgroupuser记录是组中的用户,因此在sgroupuser中不一定有数据.需要使用Left ...
Spark SQL中出现 CROSS JOIN 问题解决
Spark SQL中出现 CROSS JOIN 问题解决 1.问题显示如下所示: Use the CROSS JOIN syntax to allow cartesian products b ...
SQL中条件放在on后与where后的区别
SQL中on条件与where条件的区别数据库在通过连接两张或多张表来返回记录时,都会生成一张中间的临时表,然后再将这张临时表返回给用户. 在使用left jion时,on和where条件的区别如下: ...
SQL 中的一些小巧但常用的关键字
前面的几篇文章中,我们大体上介绍了 SQL 中基本的创建.查询语句,甚至也学习了相对复杂的连接查询和子查询,这些基本功相信你也一定掌握的不错,那么本篇则着重介绍几个技巧方面的关键字,能够让你更快更有效 ...
sql中的inner join ,left join ,right join
左连接LEFT JOIN, 也就是说,左外连接的含义是限制连接关键字右端的表中的数据必须满足连接条件,而不关左端的表中的数据是否满足连接条件,均输出左端表中的内容.不满足连接条件的 ,连接字段栏位将对 ...
关于Linq to Sql 中的left join 中defaultifempty的相关注意事项
在使用Linq to Sql的时候,进行两个表的左连接的时候要注意defaultifempty的使用,这个函数本来的意思即是:如果为空则使用默认值代替,默认值为 NULL ,当然也可以使用defaul ...
关于SQL中Union和Join的用法
转自帘卷西风的专栏(http://blog.csdn.net/ljxfblog) https://blog.csdn.net/ljxfblog/article/details/52066006 Uni ...
sql中的inner join, left join, right join的区别
下面介绍一下 inner join, left join, right join这者之间的区别现在我假设有A表和B表 left join select * from A a left join B ...
SQL中char、varchar、nvarchar、ntext的区别（转载）
char char是定长的,也就是当你输入的字符小于你指定的数目时,char(8),你输入的字符小于8时,它会再后面补空值.当你输入的字符大于指定的数时,它会截取超出的字符.nvarchar(n ...

随机推荐

RabbitMQ实战经验分享
前言最近在忙一个高考项目,看着系统顺利完成了这次高考,终于可以松口气了.看到那些即将参加高考的学生,也想起当年高三的自己. 下面分享下RabbitMQ实战经验,希望对大家有所帮助: 一.生产消息关 ...
shell crlf to lf
UNIX/Linux Commands You can use the following tools: dos2unix (also known as fromdos) – converts tex ...
vs2008单元测试
调试是解决错误的过程,测试是发现软件缺陷的过程.每一个软件在交付使用时前都必须经过测试.软件测试是软件开发的重要组成部分,现在已经发展成专门的技术. 在消除了程序中的语法错误和运行错误后,程序仍然不能 ...
Jmeter测试http+JSON配置相关
1.添加HTTP信息头管理器 Content-Type application/json Accept application/json 2.添加http请求(方法.编码.路径.body)
jmeter使用Beanshell预处理器从指定列表中获取随机值
变量mynation从列表{"china", "US", "UK"}中随机取值 String[] nation = new String[] ...
拓扑排序 --- AtCode - 3596
题目链接: https://cn.vjudge.net/problem/1137733/origin 拓扑排序的基本思想: https://blog.csdn.net/qq_41713256/arti ...
golang单例模式
1.定义:单例对象的类必须保证只有一个实例存在,全局有唯一接口访问. 2.分类: 懒汉方式:指全局的单例实例在第一次被使用时构建. 饿汉方式:指全局的单例实例在类装载时构建. 3.实现: (1)懒汉方 ...
BZOJ2319 : 黑白棋游戏
将01串按1分段,那么分析可得长度为$a$的段拼上长度为$b$的段的SG值为$a-[a\leq b]$. 设$f[i][j][k][l]$表示从后往前用了$i$个1,$j$个0,当前段长度为$k$,后 ...
ab测试工具
我们可以模拟100个并发用户,对一个页面发送1000个请求输入命令:ab -n1000 -c100 http://www.jb51.net/ 结果: ##首先是apache的版本信息 This is ...
AsyncLocal<T>与ThreadLocal<T>区别研究
using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.T ...

sql中的left join以及on、where关键字的区别

sql中的left join以及on、where关键字的区别的更多相关文章

随机推荐

热门专题