1 基础表

创建表A 表B

create table testA(id int, name varchar(10));
create table testB(id int, name varchar(10));

2 插入数据

insert into testA values(1,'zhangssa'),(2,'lisi'),(3,'wangwu')
insert into testB values(2,'zhangssa2'),(3,'lisi2'),(4,'wangwu4')

 3  left join

3.1 当条件是 1 = 1 时，执行结果和outer join没有区别

select * from testA left join testB on 1 = 1;

 3.2 当条件除了有 1 = 1之外，还有其他条件时，执行结果如下

其实这里有无 条件“1=1”，你都会发现没有区别

select A.* from testA left join B on 1 = 1 and testA.id = testB.id

select A.* from testA left join B on testA.id = testB.id