student 表：

课程表course:

1、key为null, 不关联

select * from student s left join course c on s.id = c.s_id;

2、on中过滤条件 与 where 过滤条件区别

on and c.id<>‘1001’ 先过滤右表数据，然后与左表关联

select * from student s left join course c on s.id = c.s_id and c.id<>'1001';

where c.id<>‘1001’ 先关联，然后过滤结果数据

select * from student s left join course c on s.id = c.s_id where c.id<>'1001';

模拟数据：

drop table if exists student;
create table student (
 id int,
 name STRING
);

insert into student values (1, 'chb'), (2, 'lis'), (3, 'wzng'), (4, 'sdsd'),(cast(null as int), 'sdok');



create table course(
 id STRING,
 name STRING,
 s_id int
);

insert into course values ('1001', 'c1', 1), ('1001', 'c1', 2), ('1002', 'c2', 3),('1002', 'c2', 3),('1003', 'c3', 5),('1003', 'c3', cast(null as int));