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，须要用替代的解决方案来实现INTERSECT ALL。能够用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没必要在乎行的顺序。若是想让返回的结果不包含行号，则能够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一样。

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;