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  • 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

    1 SELECT country, region, city FROM HR.Employees
    2 UNION ALL
    3 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

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

    INTERSECT(交集)集合运算

    1.INTERSECT DISTINCT集合运算

    1 SELECT country, region, city FROM HR.Employees
    2 INTERSECT
    3 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

    1 SELECT
    2   ROW_NUMBER() 
    3     OVER(PARTITION BY country, region, city
    4          ORDER     BY (SELECT 0)) AS rownum,
    5   country, region, city
    6 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:

     1 SELECT
     2   ROW_NUMBER() 
     3     OVER(PARTITION BY country, region, city
     4          ORDER     BY (SELECT 0)) AS rownum,
     5   country, region, city
     6 FROM HR.Employees
     7 
     8 INTERSECT
     9 
    10 SELECT
    11   ROW_NUMBER() 
    12     OVER(PARTITION BY country, region, city
    13          ORDER     BY (SELECT 0)),
    14   country, region, city
    15 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

    1 SELECT country, region, city FROM HR.Employees
    2 EXCEPT
    3 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:

     1 WITH EXCEPT_ALL
     2 AS
     3 (
     4   SELECT
     5     ROW_NUMBER() 
     6       OVER(PARTITION BY country, region, city
     7            ORDER     BY (SELECT 0)) AS rownum,
     8     country, region, city
     9     FROM HR.Employees
    10 
    11   EXCEPT
    12 
    13   SELECT
    14     ROW_NUMBER() 
    15       OVER(PARTITION BY country, region, city
    16            ORDER     BY (SELECT 0)),
    17     country, region, city
    18   FROM Sales.Customers
    19 )
    20 SELECT country, region, city
    21 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。

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  • 原文地址:https://www.cnblogs.com/laixiancai/p/4352597.html
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