How to I modify this t-sql query to return the max

问题:

I have the following query:

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

Instead of returning all the columns, I just want to return Rate and the Maximum value between RateMon, RateTue, RateWed, RateThu, RateFri, RateSat, and RateSun, but I am having a hard time because the column names are different.

An example result return now is:

100, 400, 400, 400, 400, 600, 600, 600

where 100 is the Rate and the other values correspond to Mon - Sun, but I want to return just 100 and 600 in this case.

回答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

回答2:

You could unpivot the days with a union subquery:

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

回答3:

You can use something verbose like:

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   

A lot of typing, but that's one possible answer. You can also do the following, which is slightly less typing:

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   

Of course, the correct solution would be to normalize your database and make this moot, since a simple join and aggregate would be sufficient.