最大的行記錄作爲一列

Question

CREATE TABLE test_max (
number_a INT, 
number_b INT, 
number_c INT, 
) 

GO 
INSERT INTO test_max VALUES(3,7,9) 
INSERT INTO test_max VALUES(5,2,10) 
INSERT INTO test_max VALUES(31,4,91) 
INSERT INTO test_max VALUES(35,124,1) 
GO 

select number_a,number_b,number_c from test_max 

我想打印第四列是MAX（number_a.value，number_b.value，number_c.value）

所以輸出應該像

3 7 9 9 
5 2 10 10 
31 4 91 91 
35 124 1 124 

是否有可能不使用PIVOT作爲列可能不只是3？

Answer 1

你可能想MAX相當於像GREATEST行MySQL/Oracle：

SELECT number_a,number_b,number_c, c.s AS greatest 
FROM #test_max 
CROSS APPLY (SELECT MAX(v) 
      FROM (VALUES (number_a),(number_b),(number_c)) AS c(v) 
      ) AS c(s) 

LiveDemo

如果您使用以下SQL Server 2008使用的版本：

SELECT number_a,number_b,number_c, c.s AS greatest 
FROM #test_max 
CROSS APPLY (SELECT MAX(v) 
      FROM (SELECT number_a AS v 
        UNION ALL SELECT number_b 
        UNION ALL SELECT number_c 
       ) AS c(v) 
      ) AS c(s); 

輸出：

╔══════════╦══════════╦══════════╦══════════╗ 
║ number_a ║ number_b ║ number_c ║ Greatest ║ 
╠══════════╬══════════╬══════════╬══════════╣ 
║  3 ║  7 ║  9 ║  9 ║ 
║  5 ║  2 ║  10 ║  10 ║ 
║  31 ║  4 ║  91 ║  91 ║ 
║  35 ║  124 ║  1 ║  124 ║ 
╚══════════╩══════════╩══════════╩══════════╝ 

它是如何工作的：

1. 具有行值表VALUES/UNION ALL
2. 創建派生表（子查詢）源自表
3. 獲取MAX值做它在每一行表 Using APPLY