如果有空，ISNULL不显示列？

Question

我有查询

SELECT [Category] = ISNULL ( c.Category,'0'),
       COUNT (cl.ClientID) as [3 Mo Clients]
FROM Category c  LEFT JOIN clients cl ON cl.CategCode = c.CategCode 
WHERE agencyid =2
GROUP BY  c.Category, c.CategCode

它的工作但它必须显示Category表中的所有类别，即使它们是null 但是当我运行我的查询时，它看起来像这样

来自类别表的

类别

Child
Infant
Newborn
Pregnant
Postpartum
Senior

任何创意方式ISNULL都必须按原样运作？

Answer 1

因为您已过滤WHERE子句中的行，导致其行为类似于INNER JOIN。您需要将条件放在ON子句中，因为我猜测在表agencyid 上找到了列CategCode。

SELECT  [Category] = ISNULL ( c.Category,'0'),
        COUNT (cl.ClientID) as [3 Mo Clients]
FROM    Category c  
        LEFT JOIN clients cl 
           ON cl.CategCode = c.CategCode  AND 
              cl.agencyid = 2
GROUP   BY  c.Category, c.CategCode

更新1

SELECT  [Category] = ISNULL ( c.Category,'0'),
        COUNT (cl.ClientID) as [3 Mo Clients],
        SUM(CASE WHEN statusid in (1,2) THEN 1 ELSE 0 END) [Status_1_2]
FROM    Category c  
        LEFT JOIN clients cl 
           ON cl.CategCode = c.CategCode  AND 
              cl.agencyid = 2
GROUP   BY  c.Category, c.CategCode