现有表table1 , 四个int型 字段 F1,F2,F3,F4；现要查询得到四列中的最大值 , 并只需要返回最大值

方法一：

select case when F12>F34 then F12 else F34 end as MaxNum from
(select case when F1>F2 then F1 else F2 end as F12,
case when F3>F4 then F3 else F4 end as F34 from table1) as t1 order by MaxNum desc

效果如下：

方法二：

SELECT TOP 1 * FROM (
SELECT F1 AS MaxNum,'F1' AS ColumnName FROM TABLE1 
UNION 
SELECT F2 AS MaxNum,'F2' AS ColumnName FROM TABLE1
UNION 
SELECT F3 AS MaxNum,'F3' AS ColumnName FROM TABLE1
UNION 
SELECT F4 AS MaxNum,'F4' AS ColumnName FROM TABLE1) AS T
ORDER BY T.MaxNum DESC 

效果如下：

小注：有4列 数据，这里 三个 union 连接的 四个 select 语句，每个查一列，最后四列数据变成一行，然后再查，就好查了。

对于方法二的知识补充：SQL UNION 和 UNION ALL 操作符