create table Customers(
Id int identity(1000000,1),
Company varchar(30) null,
Name varchar(20) null
)
insert into Customers values('Fun4All','Tom')
insert into Customers values('Alibaba','Tim')
insert into Customers values('BaiDu','Jerry')
insert into Customers values('G0ogle','Herry')
insert into Customers values('Tencent','MaHuaTeng')
insert into Customers values('Fun4All','Bill')

这是讲解所需要的sql脚本

一、自联结       代码如下:

select * from Customers

aaarticlea/png;base64,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" alt="" />

现在有个需求,需要给Tom同一公司的所有会员发送一条邮件。

分析下基本思路,首先根据Tom找到其所在的公司名,在根据公司名找到其公司民下的所有会员。下面是解决代码:

select *
from Customers
where Company in(select Company from Customers where Name='Tom')

aaarticlea/png;base64,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" alt="" />

ok,通过子查询轻松的完成需求!

但是这边有一个重点,你必须要知道,子查询虽然方便,但是大多数DBMS的处理子查询的速度比处理联结要慢的多,如果数据量少的话,几乎没什么影响,但是如果后期数据会很大的话,就应该使用自联结!

下面是使用自联结的解决代码:

select *
from Customers c1,Customers c2
where c1.Company=c2.Company

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" alt="" />

这是自联结之后的效果!

select *
from Customers c1,Customers c2
where c1.Company=c2.Company
and c2.Name='Tom'

aaarticlea/png;base64,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" alt="" />

这个是两张表(实际是一张表)联结之后的结果图,ok完成需求!

自联结通过联结的机制,将所有公司名相同的记录联结到一起,即每一个公司的会员,都会和同一公司的其他会员联结一次,因为他们的公司名相同,这是时候指定c2.name='Tom',就能拿到Tom和其公司所有会员的资料,这个时候就可以发邮件了!

二、自然联结

无论何时对表进行联结,应该至少有一列不止出现在一个表中(被联结的列)。标准的联结返回所有的数据,相同的列甚至出现多次。而自然联结就是排除多次出现,是每一列只出现一次。

但是SQL不提供这项功能,自然联结的功能是需要你自己去完成的,自然联结要求你只能选择哪些唯一的列,一般通过对一个表使用通配符(SELECT *),而对其他表的列使用明确的子集来完成。

注意:我们迄今为止建立的每个内联结都是自然联结,很可能永远都不会用到不是自然联结的内联结。

三、外联结(左外联结(LEFT OUTER JOIN)和右外联结(RIGHT OUTER JOIN))

许多联结将一个表中的行与另一个表中的行相关联,但有时候需要需要包含哪些没有关联的行。例如,有如下需求

(1)对每个顾客下的订单书进行计数,包括那些至今尚未下订单的顾客;

(2)列出所有产品以及订购数量,包括没有人订购的产皮

(3)计算平均销售规模,包括那些至今尚未下订单的顾客;

内联结代码如下:

select * from Customers
select * from Orders
select *
from Customers a INNER JOIN Orders b
ON a.Id=b.CustomerId

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" alt="" />

这边我们使用了内联结检索出所有用户的订单,通过内联结的检索结果和外联结的检索结果比较,来更好的理解外联结!

通过结果图,发现内联结确实把所有用户的订单都检索出来了,但是这里内联结似乎把没有订单的用户给过滤掉了,所以我们分析得出内联结只把有订单的所有用户信息包括订单信息检索出来,但是如果用户没有订单,则该用户信息会被过滤!

下面再来看外联结代码:

select * from Customers
select * from Orders select *
from Customers a LEFT OUTER JOIN Orders b
ON a.Id=b.CustomerId

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" alt="" />

这边我们使用了左外联结检索出所有用户的订单

通过结果图,发现外联结也把所有用户的订单都检索出来了,但是这里外联结把没有订单的用户也检索出来了(即外联结能检索出没有关联行的行,对应的用null来填充其关联行),只是它的订单信息没有,所以对应的用null来替换,这就是外联结和内联结的最主要的差别,我们可以根据不同的业务需求,来选择性的使用它们!

四、全外连接(FULL OUTER JOIN)

select * from Customers
select * from Orders
select *
from Customers a FULL OUTER JOIN Orders b
ON a.Id=b.CustomerId

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" alt="" />

分析Orders和Customers表结果图发现:赵六和冯七没有下订单,而订单表中的第四单没有与之对应的顾客,再看全连接之后的结果图,发现全联结把所有没有没有与之对应关联行的行全部用null填充,这就是全联结的作用!

五、使用带聚集函数的联结

聚集函数是用来汇总数据,在我前面的随笔中所用的聚集函数的例子都是从一个表中汇总数据,其实聚集函数也可以和联结一起使用,其实联结是多个小表组装而成的一个大表(你可以这样理解).

下面通过例子来了解其用法,代码如下:

select * from Customers
select * from Orders

aaarticlea/png;base64,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" alt="" />

现在有个需求,需要检索出所有顾客的订单数量(没有则为0),下面是解决代码:

select a.Id,a.Name,COUNT(b.CustomerId)
from Customers a LEFT JOIN Orders b
ON a.Id=b.CustomerId
GROUP by a.Name,a.Id

aaarticlea/png;base64,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" alt="" />

ok,完成需求!

六、使用联结和联结条件(使用联结主要注意的要点)

(1)注意所使用的联结类型。一般我们使用内联结(INNER JOIN)使用外联结同样有效!   ---这一点上面'三'已论述

(2)保证使用正确的联结条件(不管采用哪种语法、那种联结),否则会返回不正确的数据。

(3)所有的联结都需要提供联结条件,否则会得出笛卡尔积。

(4)在一个联结中可以包含多个表,甚至可以对每个联结采用不同的联结类型。这样做是合法的,也是有用的,但是在一起测试它们前分别测试每个联结。这样会使排除故障更简单。

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