如何修改此 t-sql 查询以返回不同列名的最大值？

Question

我有以下疑问：

SELECT
        [Rate],
        [RateMon],
        [RateTue],
        [RateWed],
        [RateThu],
        [RateFri],
        [RateSat],
        [RateSun]
    FROM
        [Room]
    WHERE
        Id=@Id

我不想返回所有列，而只想返回 Rate 和 RateMon、RateTue、RateWed、RateThu、RateFri、RateSat 和 RateSun 之间的最大值，但我很难，因为列名是不同。

现在返回的示例结果是：

100、400、400、400、400、600、600、600

其中 100 是速率，其他值对应于周一 - 周日，但在这种情况下我只想返回 100 和 600。

Answer 1

SELECT  [Rate],
        (SELECT MAX(T.[Rate])
         FROM (VALUES([RateMon]),
                     ([RateTue]),
                     ([RateWed]),
                     ([RateThu]),
                     ([RateFri]),
                     ([RateSat]),
                     ([RateSun])) AS T([Rate])
        ) AS MaxRate
FROM [Room]
WHERE Id=@Id

Answer 2

您可以使用 union 子查询来反透视：

select  Rate
,       max(DayRate)
from    (
        select ID, Rate, RateMon as DayRate from Room
        union all
        select ID, Rate, RateTue from Room
        union all
        select ID, Rate, RateWed from Room
        union all
        ....
        ) as SubQuery
where   ID = @ID
group by
        Rate

Answer 3

你可以使用一些详细的东西，比如：

SELECT Rate,
  CASE 
    WHEN RateMon>=RateTue AND RateMon>=RateWed AND RateMon>=RateThu AND 
         RateMon>=RateFri AND RAteMon>=RateSat AND RateMon>=RateSun THEN RateMon
    WHEN RateTue>=RateMon AND RateTue>=RateWed AND RateTue>=RateThu AND 
         RateTue>=RateFri AND RateTue>=RateSat AND RateTue>=RateSun THEN RateTue
    WHEN RateWed>=RateMon AND RateWed>=RateTue AND RateWed>=RateThu AND 
         RateWed>=RateFri AND RateWed>=RateSat AND RateWed>=RateSun THEN RateWed
    WHEN RateThu>=RateMon AND RateThu>=RateTue AND RateThu>=RateWed AND 
         RateThu>=RateFri AND RateThu>=RateSat AND RateThu>=RateSun THEN RateThu
    WHEN RateFri>=RateMon AND RateFri>=RateTue AND RateFri>=RateWed AND 
         RateFri>=RateThu AND RateFri>=RateSat AND RateFri>=RateSun THEN RateFri
    WHEN RateSat>=RateMon AND RateSat>=RateTue AND RateSat>=RateWed AND 
         RateSat>=RateThu AND RateSat>=RateFri AND RateSat>=RateSun THEN RateSat
    WHEN RateSun>=RateMon AND RateSun>=RateTue AND RateSun>=RateWed AND 
         RateSun>=RateThu AND RateSun>=RateFri AND RateSun>=RateSat THEN RateSun
    END AS MaxRate 
FROM    
    [Room]    
WHERE    
    Id=@Id   

打字很多，但这是一种可能的答案。您还可以执行以下操作，这会稍微减少输入：

SELECT Rate, MAX(Rates.Rate) AS MaxRate
  FROM    
    [Room], 
      (SELECT RateMon AS Rate FROM [Room] WHERE Id=@Id UNION
       SELECT RateTue AS Rate FROM [Room] WHERE Id=@Id UNION
       SELECT RateWed AS Rate FROM [Room] WHERE Id=@Id UNION
       SELECT RateThu AS Rate FROM [Room] WHERE Id=@Id UNION
       SELECT RateFri AS Rate FROM [Room] WHERE Id=@Id UNION
       SELECT RateSat AS Rate FROM [Room] WHERE Id=@Id UNION
       SELECT RateSun AS Rate FROM [Room] WHERE Id=@Id)
     AS Rates
  WHERE    
    Id=@Id   

当然，正确的解决方案是规范化您的数据库并使其没有实际意义，因为简单的连接和聚合就足够了。