1、大多数的SQL查询只包含从一个或多个表中返回数据的单条SELECT语句,但是,SQL也允许执行多个查询(多条SELECT语句),并将结果作为一个查询结果集返回。这些组合查询通常称为并或复合查询。

主要有两种情况需要使用组合查询:

(1)在一个查询中从不同的表返回结构数据

(2)对一个执行多个查询,按一个查询返回数据

2、使用UNION

使用UNION很简单,所要做的只是给出每条SELECT语句,然后再每条SELECT语句之间加上UNION关键字,这样所给出的SELECT结果集就能组合成一个结果集并返回。代码如下:

select * from dbo.Customers_1

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" 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现在有个需求,需要检索出位于浙江和上海,或者顾客名称为Fun4All的所有顾客,下面是通过WhERE子句完成的解决代码:

select *
from dbo.Customers_1
where Province in('浙江','上海') or Company='Fun4All'

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

ok,通过Where子句的OR操作符完成需求!

下面通过组合查询UNION操作符来完成需求,代码如下:

select *
from dbo.Customers_1
where Province in('浙江','上海')
select *
from dbo.Customers_1
where Company='Fun4All'

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

这是没有使用UNION的单独查询,一共有6条记录,如果是普通的结果集组合的话会出现6条记录,我们发现其中有两条记录是重复的aaarticlea/png;base64,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" alt="" />

在看使用了UNION组合查询关键字的查询解决代码:

select *
from dbo.Customers_1
where Province in('浙江','上海')
UNION
select *
from dbo.Customers_1
where Company='Fun4All'

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

ok,完成需求,通过和上面没有使用UNION关键的分开查询相比,我们发现UNION(组合查询)从结果集中去除了重复的行。

这里我们可以使用UNION ALL,告诉DBMS不取消重复的行。那么效果就和上面没加UNION的一样

3、总结

分析上面完成需求的连个例子,我们发现对于上面的简单的例子使用UNION似乎比WHERE子句更加的复杂。但是这里你需要知道,如果对于较复杂的过滤条件,或者从多个表(而不是一个表)中检索数据的情形,使用UNION可能会使处理更加的简单。

4、使用UNION必须知道的规则(使用注意事项)

(1)使用UNION必须有两条或者两条以上的SELECT语句组成,语句之间用UNION关键字分割

(2)使用UNION关联的每个子查询必须包含相同的检索列、表达式或这聚集函数(次序可以不一样)

(3)列数据类型必须兼容;类型不必完全相同,但必须是DBMS可以隐含转换的类型(不同的数值类型或者不同的日期类型)

(4)UNION几乎总是完成与多个WHERE条件相同的工作,UNION ALL是UNION的一种形式,它完成WHERE子句完成不了的工作,因为他将返回每个条件的匹配行(包括重复行)

(5)使用组合查询,当需要对结果进行排序是,只能指定一条Order By语句,这条语句只能放在最后一天SELECT语句的后面.

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