在 GROUP BY 中组合组答案

【问题标题】：Combining groups in GROUP BY在 GROUP BY 中组合组
【发布时间】：2013-08-13 20:08:35
【问题描述】：

我有一个按字段描述分组的查询。（总共有 9 个描述。）我需要以某种方式将其中的 3 个描述组合成一组，总共有 7 个描述。我是 sql 查询的新手，所以到目前为止，除了 JOINS 和 WHERE 子句之外，我对任何事情都很迷茫。感谢您的帮助。

SELECT        serviceclass.description, DATEPART(Year, meterread.readdate) AS Year, DATEPART(Month, meterread.readdate) AS Month, SUM(meterread.consumption) 
                     AS Consumption, COUNT(DISTINCT accountservice.account_id) AS Locations
FROM            accountservice INNER JOIN
                     serviceclass ON accountservice.serviceclass_id = serviceclass.serviceclass_id INNER JOIN
                     meterread ON accountservice.accountservice_id = meterread.accountservice_id
WHERE        (@Year = DATEPART(Year, meterread.readdate))
GROUP BY serviceclass.description, DATEPART(Year, meterread.readdate), DATEPART(Month,meterread.readdate)
ORDER BY Year, Month

【问题讨论】：

提供示例描述以及您希望如何对您感兴趣的 3 个进行分组。
您是否总是希望将相同的三个描述组合在一起，或者这三个描述可以不同？

标签： sql sql-server-2008 reporting-services

【解决方案1】：

您可以通过使用CASE 语句来实现此目的。

将SELECT 和GROUP BY 子句中的serviceclass.desription 替换为

CASE
    WHEN serviceclass.description IN ('Desc1','Desc2','Desc3')
        THEN 'JointDescription'
    ELSE serviceclass.description
END

并使用您的三个描述更新 Desc 1 ~ 3，并将联合描述更新为您想要的。

结果：

SELECT CASE
           WHEN serviceclass.description IN ('Desc1','Desc2','Desc3')
               THEN 'JointDescription'
           ELSE serviceclass.description
       END as description, 
       DATEPART(Year, meterread.readdate) AS Year,  
       DATEPART(Month, meterread.readdate) AS Month, 
       SUM(meterread.consumption) AS Consumption, 
       COUNT(DISTINCT accountservice.account_id) AS Locations
FROM accountservice 
INNER JOIN serviceclass ON accountservice.serviceclass_id = serviceclass.serviceclass_id 
INNER JOIN meterread ON accountservice.accountservice_id = meterread.accountservice_id
WHERE        (@Year = DATEPART(Year, meterread.readdate))
GROUP BY CASE
             WHEN serviceclass.description IN ('Desc1','Desc2','Desc3')
                  THEN 'JointDescription'
             ELSE serviceclass.description
         END, 
         DATEPART(Year, meterread.readdate), 
         DATEPART(Month,meterread.readdate)
ORDER BY Year, Month

【讨论】：

【解决方案2】：

您通常会使用case 语句来执行此操作，该语句将同时出现在select 和group by 语句中。比如：

select (case when serviceclass.description = 'A' then 'B'
             else serviceclass.description
        end) as description
       . . . -- fill in with query here
group by (case when serviceclass.description = 'A' then 'B'
             else serviceclass.description
           end), . . .  -- rest of query goes here

【讨论】：