sql 表连接on后面加where和and的区别

测试数据

create table testA( Id number(10) primary key, Name varchar(10) ) insert into testA values(1,'小黄'); insert into testA values(2,'小绿'); insert into testA values(3,'小白'); insert into testA values(4,'小黑'); insert into testA values(5,'小花'); commit create table testB( Id number(10) primary key, age number(10) ) insert into testB values(1,10); insert into testB values(2,11); insert into testB values(3,12); insert into testB values(4,13); commit 

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on后面加and

select * from testA a left join testB b on a.id = b.id and b.age=10 

on后面加where

select * from testA a left join testB b on a.id = b.id where b.age=10 

on 是用于消除笛卡儿积的，表连接时不加on会报错，left join语句会从左表那里返回所有的行，即使在右表中没有匹配的行，on后面接and也会兼顾左连接，不管and 后面接什么内容，左表数据都会全部展示
下图语句由于a.name =10在testA没有匹配数据，所以与其关联的testB也匹配不到数据。

select * from testA a left join testB b on a.id = b.id and a.name='10' 

使用where就是对连接后的结果集进行条件筛选

select * from testA a left join testB b on a.id = b.id where a.name='10'