筆記-Microsoft SQL Server 2008技術內幕：T-SQL語言基礎-06 集合運算

T-SQL支持3種集合運算：並集（UNION）、交集（INTERSECT）和差集（EXCEPT）。集合運算涉及的兩個查詢不能包含ORDER BY子句。

UNION ALL集合運算

UNION ALL不會對行進行比較，也不會刪除重複行。假設查詢Query1返回m行，查詢Query2返回n行，則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集合運算

UNION DISTINCT會刪除重複行。

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

INTERSECT DISTINCT集合運算

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

 注意一點，集合運算對行進行比較時，認爲兩個NULL值相等。

INTERSECT ALL集合運算

SQL Server不支持內建的INTERSECT ALL運算，須要用替代的解決方案來實現INTERSECT ALL。能夠用ROW_NUMBER來實現此需求。

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;

 執行結果：

注意上面的SQL中，在排序函數的OVER子句中使用ORDER BY(SELECT<常量>)是告訴SQL Server沒必要在乎行的順序。若是想讓返回的結果不包含行號，則能夠在這個查詢基礎上定義一個表表達式，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集合運算

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

 注意，在EXCEPT集合運算中，交換兩個集合的運算位置會使運算結果不一樣。

EXCEPT ALL運算的替代解決方案

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;