sql - LEFT JOIN 根据 WHERE 条件的位置给出不同的数据集

Question

请参阅以下 3 个要点，只是为了让您对表格数据有一个简单的了解。全部skCitizen在[dbo].[LUEducation]存在于 [dbo].[LUCitizen]

SELECT  COUNT(*) FROM   [dbo].[LUCitizen] --115000 ROWS
SELECT  COUNT(*) FROM   [dbo].[LUEducation]  --201846 ROWS

SELECT  COUNT(*) --212695 ROWS
FROM    [dbo].[LUCitizen] C
LEFT JOIN   [dbo].[LUEducation] E
ON      C.skCitizen = E.skCitizen

SELECT  COUNT(*) FROM   [dbo].[LUEducation] WHERE skSchool = 24417 --4 ROWS

<小时/>

请参阅以下 2 个查询，

SELECT  C.skCitizen,E.skCitizen
FROM    [dbo].[LUCitizen] C
LEFT JOIN   [dbo].[LUEducation] E
ON      C.skCitizen = E.skCitizen
WHERE   E.skSchool = 24417
--4 ROWS

SELECT  C.skCitizen,E.skCitizen
FROM    [dbo].[LUCitizen] C
LEFT JOIN   (SELECT * FROM [dbo].[LUEducation] WHERE skSchool = 24417) E
ON      C.skCitizen = E.skCitizen
--115000 ROWS

在最后两个查询中，令我困惑的查询是第一个。我预计115000 rows ，但只有4 rows显示。 据我了解，完整行来自 [dbo].[LUCitizen]将显示，然后从 [dbo].[LUEducation] 开始 4 行将是LEFT已加入。

为什么两个查询不同？

如果这是一个重复的问题，请原谅。

Answer 1

当你这样做时:

SELECT  C.skCitizen,E.skCitizen
FROM    [dbo].[LUCitizen] C
LEFT JOIN   [dbo].[LUEducation] E
ON      C.skCitizen = E.skCitizen
WHERE   E.skSchool = 24417;

您正在将左连接转换为内连接，因为对于非连接，E.skSchool为NULL - 匹配行。在左连接中的第二表上放置条件的正确方法是使用on子句:

SELECT  C.skCitizen,E.skCitizen
FROM    [dbo].[LUCitizen] C
LEFT JOIN   [dbo].[LUEducation] E
ON      C.skCitizen = E.skCitizen AND E.skSchool = 24417;