Question

对于作业我必须添加三列表格（基本，附加人工和附加部分）以进行全面收费。然后我还必须确定找到的最小和最大总电量以及平均值。我成功地编写了一个总体充电查询

SELECT Service.ServiceId, Sum([BasicCharges]+[AdditionalLabourCharges]+
    [AdditionalPartCharges]) AS [OverallCharge] 
FROM Service

但是，我无法理解这些min，max和avg语句。我的草稿看起来像......但不起作用

SELECT  MIN(OverallCharge) AS [MinOverallCharge], 
    MAX(OverallCharge) AS [MaxOverallCharge], 
    AVG(OverallCharge) AS [AverageOverallCharge]
FROM Service
WHERE Service
(SELECT Sum([S.BasicCharges]+[S.AdditionalLabourCharges]+[S.AdditionalPartCharges]) AS [OverallCharge]
FROM Service AS S);

非常感谢任何帮助。感谢

尼克

Answer 1

SELECT --get the min/avg/max of all charges
   MIN(OverallCharge) AS [MinOverallCharge], 
   MAX(OverallCharge) AS [MaxOverallCharge], 
   AVG(OverallCharge) AS [AverageOverallCharge]
FROM 
 ( -- calculate OverallCharge for each ServiceId
   SELECT S.ServiceId, 
      Sum([S.BasicCharges]+[S.AdditionalLabourCharges]+[S.AdditionalPartCharges]) AS [OverallCharge]
   FROM Service AS S
   GROUP BY S.ServiceId
 ) dt;

Answer 2

在这种情况下，子查询可以解决问题。使用您的第一个查询来获取每行的总和。然后你可以计算AVG，MIN和MAX：

SELECT
    MIN(subquery.OverallCharge) AS [MinOverallCharge], 
    MAX(subquery.OverallCharge) AS [MaxOverallCharge], 
    AVG(subquery.OverallCharge) AS [AverageOverallCharge]
FROM
(
    SELECT ([S.BasicCharges]+[S.AdditionalLabourCharges]+[S.AdditionalPartCharges])
        AS [OverallCharge]
    FROM Service AS S
) subquery;

Answer 3

不要忘记GROUP BY：

SELECT  MIN(OverallCharge) AS [MinOverallCharge], 
    MAX(OverallCharge) AS [MaxOverallCharge], 
    AVG(OverallCharge) AS [AverageOverallCharge]
FROM 
    (SELECT ServiceId, Sum([S.BasicCharges]+[S.AdditionalLabourCharges]+
    [S.AdditionalPartCharges]) AS [OverallCharge]
    FROM Service AS S
    GROUP BY ServiceId) dt;

子查询为每个OverallCharge生成一个包含ServiceId的表格，然后主要查询获得min，max，avg。

Subquerying的问题

3 个答案: