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个计数,只是为了简要介绍一下表格数据.所有skCitizen的[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

---

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2个查询中,对我来说令人困惑的查询是第一个.在那里,我期待115000 rows,但只4 rows显示.根据我的理解,[dbo].[LUCitizen]将显示Full rows from ,然后[dbo].[LUEducation]将有4行来自LEFTJoined.

为什么2个查询不同？

请原谅,如果这是一个重复的问题.

Answer 1

当你这样做:

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

您正在转动left join到inner join,因为E.skSchool是NULL针对非匹配行.将条件放在a中的第二个表的正确方法left join是使用以下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;

Answer 2

如果left join未能在 中找到匹配项E，则列会E收到一个null值。然后where条款：

E.skSchool = 24417

变成：

null = 24417

这不是真的。所以它会过滤掉所有行。