SQL之集合运算

UNION(并集)集合运算

1.UNION ALL集合运算

该集合运算返回在输入的多集中出现的所有行，它实际上不会对行进行比较，也不会删除重复行。假设查询Query1返回m行，查询Query2返回n行，则该集合运算后返回(m+n)行

SELECT country, region, city FROM HR.Employees
UNION ALL
SELECT country, region, city FROM Sales.Customers;

2.UNION DISTINCT集合运算

此集合运算如果两个输入集中包含相同的行，则该行在结果中只出现一次，也就是说运算结果是一个真正的集合，而不是多集

SELECT country, region, city FROM HR.Employees
UNION
SELECT country, region, city FROM Sales.Customers;

INTERSECT(交集)集合运算

1.INTERSECT DISTINCT集合运算

SELECT country, region, city FROM HR.Employees
INTERSECT
SELECT country, region, city FROM Sales.Customers;

2.INTERSECT ALL集合运算

SQL Server不支持内建的INTERSECT ALL运算，那么我们需要另外的解决方案来实现此集合运算。首先先看一段代码

SELECT
  ROW_NUMBER() 
    OVER(PARTITION BY country, region, city
         ORDER     BY (SELECT 0)) AS rownum,
  country, region, city
FROM HR.Employees

执行结果：

ROW_NUMBER函数是关键，它能给我们提供行号；另外，上面的SQL中ORDER BY后面的SELECT 0是表示查询可以不必在意行的顺序，即不对数据进行排序。分析该查询结果可知，INTERSECT ALL的完整解决方案是：

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;

 EXCEPT(差集)集合运算

注意，集合A与B的差集(A-B)是由属于集合A，但不属于集合B的元素组成的集合，也就是两集和之差A-B是从A中减去B中也属于A的元素，故差集运算是不对称的，A-B和B-A是不一样的。

1.EXCEPT DISTINCT集合运算

SELECT country, region, city FROM HR.Employees
EXCEPT
SELECT country, region, city FROM Sales.Customers;

 2.EXCEPT ALL集合运算

SQL Server没有提供内建的此集合运算。解决方案如下：

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;

 集合运算的优先级：INTERSECT比UNION和EXCEPT运算的优先级高，UNION和EXCEPT优先级相等。