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

---

on后面加and

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

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on后面加where

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

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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'

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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' 

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