sql中外连接条件位置不同导致的查询结果不过

 今天研究了两个表中外连接条件位置不同，导致的查询结果不同，

 
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---DROP TABLE #t1,#t2 
 
---创建表1 
CREATE TABLE #t1(areaid NVARCHAR(20),housecode NVARCHAR(20),expressid INT) 
 
INSERT INTO #t1(areaid, housecode, expressid )VALUES  ('101','01', 1) 
INSERT INTO #t1(areaid, housecode, expressid )VALUES  ('101','02', 1) 
INSERT INTO #t1(areaid, housecode, expressid )VALUES  ('101','03', 1) 
INSERT INTO #t1(areaid, housecode, expressid )VALUES  ('101','04', 1) 
 
--创建表2 
CREATE TABLE #t2(areaid NVARCHAR(20),housecode NVARCHAR(20),expressid INT) 
 
INSERT INTO #t2(areaid, housecode, expressid )VALUES  ('101','01', 0) 
INSERT INTO #t2(areaid, housecode, expressid )VALUES  ('101','02', 1) 
INSERT INTO #t2(areaid, housecode, expressid )VALUES  ('101','03', 0) 
INSERT INTO #t2(areaid, housecode, expressid )VALUES  ('101','04', 1) 
 
---#t2.expressid=0跟在where后面 
SELECT #t1.*,#t2.* FROM #t1 LEFT JOIN #t2  
ON #t1.areaid = #t2.areaid AND #t1.housecode = #t2.housecode   
WHERE  #t2.expressid=0 
 
---#t2.expressid=0跟在on后面 
SELECT #t1.*,#t2.* FROM #t1 LEFT JOIN #t2  
ON #t1.areaid = #t2.areaid AND #t1.housecode = #t2.housecode  AND #t2.expressid=0   

两种查询结果对比