笔记-Microsoft SQL Server 2008技术内幕：T-SQL语言基础-06 集合运算

T-SQL支持3种集合运算：并集（UNION）、交集（INTERSECT）和差集（EXCEPT）。集合运算涉及的两个查询不能包含ORDER BY子句。

UNION ALL集合运算

UNION ALL不会对行进行比较，也不会删除重复行。假设查询Query1返回m行，查询Query2返回n行，则Query1 UNION ALL Query2返回（m+n）行。

SELECT country, region, city FROM HR.Employees

UNION ALL

SELECT country, region, city FROM Sales.Customers;

UNION DISTINCT集合运算

UNION DISTINCT会删除重复行。

SELECT country, region, city FROM HR.Employees

UNION

SELECT country, region, city FROM Sales.Customers;

INTERSECT DISTINCT集合运算

SELECT country, region, city FROM HR.Employees

INTERSECT

SELECT country, region, city FROM Sales.Customers;

 注意一点，集合运算对行进行比较时，认为两个NULL值相等。

INTERSECT ALL集合运算

SQL Server不支持内建的INTERSECT ALL运算，需要用替代的解决方案来实现INTERSECT ALL。可以用ROW_NUMBER来实现此需求。

SELECT

  ROW_NUMBER() 

    OVER(PARTITION BY country, region, city

         ORDER     BY (SELECT 0)) AS rownum,

  country, region, city

FROM HR.Employees



INTERSECT



SELECT

  ROW_NUMBER() 

    OVER(PARTITION BY country, region, city

         ORDER     BY (SELECT 0)),

  country, region, city

FROM Sales.Customers;

 执行结果：

注意上面的SQL中，在排序函数的OVER子句中使用ORDER BY(SELECT<常量>)是告诉SQL Server不必在意行的顺序。如果想让返回的结果不包含行号，则可以在这个查询基础上定义一个表表达式，如：

WITH INTERSECT_ALL

AS

(

  SELECT

    ROW_NUMBER() 

      OVER(PARTITION BY country, region, city

           ORDER     BY (SELECT 0)) AS rownum,

    country, region, city

  FROM HR.Employees



  INTERSECT



  SELECT

    ROW_NUMBER() 

      OVER(PARTITION BY country, region, city

           ORDER     BY (SELECT 0)),

    country, region, city

  FROM Sales.Customers

)

SELECT country, region, city

FROM INTERSECT_ALL;

EXCEPT DISTINCT集合运算

SELECT country, region, city FROM HR.Employees

EXCEPT

SELECT country, region, city FROM Sales.Customers;

 注意，在EXCEPT集合运算中，交换两个集合的运算位置会使运算结果不同。

EXCEPT ALL运算的替代解决方案

WITH EXCEPT_ALL

AS

(

  SELECT

    ROW_NUMBER() 

      OVER(PARTITION BY country, region, city

           ORDER     BY (SELECT 0)) AS rownum,

    country, region, city

    FROM HR.Employees



  EXCEPT



  SELECT

    ROW_NUMBER() 

      OVER(PARTITION BY country, region, city

           ORDER     BY (SELECT 0)),

    country, region, city

  FROM Sales.Customers

)

SELECT country, region, city

FROM EXCEPT_ALL;