SQL Server用Cross ApplyCross Join实现列转行

Posted 2022-06-08 詩和遠方

今天介绍4种列转行（又称宽表转长表、逆透视等）写法。

数据准备

CREATE TABLE Sales([Year] INT,Q1 INT,Q2 INT,Q3 INT,Q4 INT);
GO
INSERT INTO Sales([Year],Q1,Q2,Q3,Q4)
    VALUES (2017,12,123,324,123)
          ,(2018,13,455,324,878)
          ,(2019,29,786,473,633);

SQL写法

UNION

最传统最通用写法

SELECT [Year],'Q1' AS [Quarter],Q1 AS Amount FROM Sales UNION ALL
SELECT [Year],'Q2' AS [Quarter],Q2 AS Amount FROM Sales UNION ALL
SELECT [Year],'Q3' AS [Quarter],Q3 AS Amount FROM Sales UNION ALL
SELECT [Year],'Q4' AS [Quarter],Q4 AS Amount FROM Sales; 

UNPIVOT

unpivot 的独特语法：

SELECT [Year],[Quarter],[Amount]
FROM Sales AS a
UNPIVOT(Amount FOR [Quarter] IN (Q1,Q2,Q3,Q4)) AS b;

CROSS JOIN

其实就是笛卡尔积，用CROSS JOIN是MSSQL的推荐写法，其它数据库若不支持此语法，可以将CROSS JOIN替换为逗号，或用Join On 1 = 1

SELECT a.[Year],b.[Quarter]
      ,CASE b.[Quarter] 
           WHEN 'Q1' THEN Q1
           WHEN 'Q2' THEN Q2
           WHEN 'Q3' THEN Q3
           WHEN 'Q4' THEN Q4
       END AS Amout
FROM Sales a
CROSS JOIN (VALUES ('Q1'),('Q2'),('Q3'),('Q4')) AS b([Quarter]);

CROSS APPLY

CROSS APPLY是个强大的操作符，也可以用来实现unpivot：

SELECT a.[Year], b.[Quarter],b.Amount
FROM Sales a
CROSS APPLY (SELECT 'Q1' AS [Quarter],a.Q1 AS Amount UNION ALL 
             SELECT 'Q2',a.Q2 UNION ALL 
             SELECT 'Q3',a.Q3 UNION ALL 
             SELECT 'Q4',a.Q4) AS b;
-- 或
SELECT a.[Year], b.[Quarter],b.Amount
FROM Sales a
CROSS APPLY (VALUES ('Q1',a.Q1)
                   ,('Q2',a.Q2)
                   ,('Q3',a.Q3)
                   ,('Q4',a.Q4)) AS b([Quarter],Amount);

总结与说明

• 传统的Union写法需要扫描多次基表，效率最差
• UNPIVOT写法最简洁，而且执行效率高，但语法有点晦涩，不是SQL的典型风格
• CROSS JOIN与CROSS APPLY写法也只要扫描一次基表，可能执行效率比UNPIVOT差一点点，但好处是语法上更灵活