2表中的左连接

为什么这个查询不能正确执行？2表中的左连接

SELECT pr.ProjectNumber, 
    SUM(ma.TotalAmount) As CostOfMaterials, 
    SUM(ot.TotalAmount) AS CostOfOthers 
FROM [dbo].[tblProject] AS pr 
LEFT JOIN [dbo].[tblCostOfMaterials] AS ma ON pr.ProjectNumber=ma.ProjectNumber 
LEFT JOIN [dbo].[tblCostOfOthers] AS ot ON pr.ProjectNumber=ot.ProjectNumber 
GROUP BY pr.ProjectNumber

来源

2011-02-17 jack2000

您是否收到错误？如果是这样，错误是什么意思？如果你只是没有得到你想要的结果，你希望达到什么目的，这会给你什么？我们需要更多信息来帮助你。 – anothershrubery 2011-02-17 12:03:58

为什么这个问题不正确？没有某种错误信息，很难提供帮助。 – Dai 2011-02-17 12:04:37

我假定你的意思是说，因为每个表有多个行的特定项目编号，你可以多次计算相同的值。为了避免这种情况，你可以使用

;WITH ma 
    AS (select ProjectNumber, 
       SUM(TotalAmount) as CostOfMaterials 
     FROM [dbo].[tblCostOfMaterials] 
     GROUP BY ProjectNumber), 
    ot 
    AS (select ProjectNumber, 
       SUM(TotalAmount) as CostOfOthers 
     FROM [dbo].[tblCostOfOthers] 
     GROUP BY ProjectNumber) 
SELECT pr.ProjectNumber, 
     CostOfMaterials, 
     CostOfOthers 
FROM [dbo].[tblProject] AS pr 
     LEFT JOIN ma 
     ON pr.ProjectNumber = ma.ProjectNumber 
     LEFT JOIN ot 
     ON pr.ProjectNumber = ot.ProjectNumber

为什么你这种行为的原因是因为你越来越小直角连接

WITH tblProject (ProjectNumber) AS 
(
SELECT 1 
),tblCostOfMaterials(ProjectNumber, TotalAmount) AS 
(
SELECT 1,101 UNION ALL 
SELECT 1,201 UNION ALL 
SELECT 1,301 
), 
tblCostOfOthers(ProjectNumber, TotalAmount) AS 
(
SELECT 1,100 UNION ALL 
SELECT 1,200 
) 
SELECT * 
FROM [tblProject] AS pr 
LEFT JOIN [tblCostOfMaterials] AS ma ON pr.ProjectNumber=ma.ProjectNumber 
LEFT JOIN [tblCostOfOthers] AS ot ON pr.ProjectNumber=ot.ProjectNumber

ProjectNumber ProjectNumber TotalAmount ProjectNumber TotalAmount 
------------- ------------- ----------- ------------- ----------- 
1    1    101   1    100 
1    1    101   1    200 
1    1    201   1    100 
1    1    201   1    200 
1    1    301   1    100 
1    1    301   1    200

你可以看到，值由另一个表中的匹配行数重复，因此SUM将不正确。

来源

2011-02-17 12:07:24

2表中的左连接

回答

相关问题