SQL查询从具有Null值的表中找出最大数目

Question

How Can I work out this requirement. Please help.

Client Table - CT

ClientID      Balance  
123             10
123             20  
123             30
123             40  
124             50
124             60  
124             Null  

I want to find our the max(Balance) from the CT Table.
Condition - > If there is no null value then I have to find out max(Balance) otherwise It should be Null. See below result, that am expecting.

ClientID     Balance  
123           40  
124           Null

Am writing the query as below. But Is there any more dynamic way to do?

Select ClientID,  
       CASE WHEN MIN(Balance) = NULL THEN  
                 NULL  
             ELSE  
                 MAX(Balance) END AS 'MaxBalance'  
From CT  
Group by clientID  

Please let me know, is there any otheralternative?

Answer 1

Try this:

Select ClientID,
       (CASE WHEN count(balance) < count(*)
             THEN NULL 
             ELSE MAX(Balance)
        END) AS MaxBalance
From CT
Group by clientID  

Or, a bit more cumbsersome, but perhaps clearer:

Select ClientID,
       (CASE WHEN sum(case when balance is null then 1 else 0 end) > 0
             THEN NULL 
             ELSE MAX(Balance)
        END) AS MaxBalance
From CT
Group by clientID  

Answer 2

How about:

SELECT clientid
    , balance
FROM 
    ( 

        SELECT clientid
            , balance
            , row_number()
                over(   partition by clientid
                    order by CASE WHEN balance IS NULL THEN 0 ELSE 1 END
                        , balance DESC
                ) r
        FROM ct
    ) n 
WHERE r = 1

Answer 3

I'm not sure what datatype [Balance] is, but if it is an int, you can do the following:

Select ClientID, NULLIF(MAX(ISNULL(Balance,2147483647)),2147483647)
From CT 
GROUP BY ClientID

If [Balance] is not an int, just replace 2147483647 with the max value of that datatype.

The danger, of course, would be if you really do have a client with a balance of 2147483647. In such a case their max balance would show as null.

Answer 4

This will work on SQL 2005+

; WITH a AS (       
SELECT DISTINCT ClientID
FROM CT 
WHERE Balance IS NULL   )     

SELECT t.ClientID, MAX(t.Balance) "MaxBalance"
FROM CT t
    LEFT JOIN a ON t.CLientID = a.ClientID
WHERE a.ClientID IS NULL
GROUP BY t.CLientID
UNION ALL
SELECT a.ClientID, NULL
FROM a

Answer 5

This works but is not very elegant:

SELECT SUB.ClientID, CASE (SELECT COUNT(ClientID) 
                           FROM MyTable MT 
                           WHERE Balance IS NULL 
                           AND MT.ClientID = SUB.ClientID)
                    WHEN 0 THEN (SELECT MAX(Balance) 
                                 FROM MyTable MT 
                                 WHERE MT.ClientID = SUB.ClientID)
                    ELSE NULL END AS Balance
FROM (SELECT ClientID 
      FROM [MyTable] 
      GROUP BY ClientID) SUB