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

DNS信息收集工具dig使用
Dig是域信息搜索器的简称(Domain Information Groper),使用dig命令可以执行查询域名相关的任务常见域名记录: A(主机记录把一个域名解析成IP地址) C name(别名 ...
spring_AOP
例子代码理解AOP AOP为Aspect Oriented Programming的缩写,意为:面向切面编程.大概意思就是在原有源代码的基础上,增加功能,而又不修改原有的代码. 术语切面(Aspe ...
Oracle date-time
Name Description ADDDATE() Add time values (intervals) to a date value ADDTIME() Add time CONVERT_TZ ...
在控制台下玩玩dotnet core内置原生的DI
转载请注明出处:http://www.cnblogs.com/zhiyong-ITNote/ 在基于dotnet core的web开发中,我们会经常用到DI,那么如果单单使用dotnet core自身 ...
SpringBoot集成阿里巴巴Druid监控
druid是阿里巴巴开源的数据库连接池,提供了优秀的对数据库操作的监控功能,本文要讲解一下springboot项目怎么集成druid. 本文在基于jpa的项目下开发,首先在pom文件中额外加入drui ...
Xamarin Essentials教程数据处理传输数据
Xamarin Essentials教程数据处理传输数据在移动应用程序中,除了常规的数据处理,还涉及数据存储.数据传输.版本数据多个方面.Xamarin.Essentials组件提供了多个数据处理相 ...
C# 中BindingSource 的用法
.引言 BindingSource组件是数据源和控件间的一座桥,同时提供了大量的API和Event供我们使用.使用这些API我们可以将Code与各种具体类型数据源进行解耦:使用这些Event我们可以洞 ...
Codeforces.954I.Yet Another String Matching Problem(FFT)
题目链接 \(Description\) 对于两个串\(a,b\),每次你可以选择一种字符,将它在两个串中全部变为另一种字符. 定义\(dis(a,b)\)为使得\(a,b\)相等所需的最小修改次数. ...
Django 自定义模板语法
from django import template from django.utils.safestring import mark_safe register = template.Librar ...
[USACO18DEC]Cowpatibility
Description: Farmer John 的 \(N\) 头奶牛(\(2\le N\le 5\times 10^4\))各自列举了她们最喜欢的五种冰激凌口味的清单.为使这个清单更加精炼,每种可 ...

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