Question

我如何解决此要求。请帮忙。

客户表 - CT

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

我想从CT表中找到最大值（平衡值）条件 - ＆gt;如果没有空值，那么我必须找出max（Balance）否则它应该是Null。见下面的结果，我期待。

ClientID     Balance  
123           40  
124           Null

我写的查询如下。但还有更有活力的方法吗？

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

请让我知道，还有其他选择吗？

Answer 1

试试这个：

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

或者，更多的是cumbsersome，但也许更清楚：

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

怎么样：

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

我不确定[Balance]的数据类型是什么，但如果是int，则可以执行以下操作：

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

如果[Balance]不是int，只需将2147483647替换为该数据类型的最大值。

当然，如果你确实拥有余额为2147483647的客户，则存在危险。在这种情况下，他们的最大余额将显示为空。

Answer 4

这适用于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

这可行，但不是很优雅：

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

SQL查询从具有Null值的表中查找最大数量

5 个答案: