1、内连接 inner join
JOIN 相当于INNER JOIN内连接，不满足on 的条件过滤掉，不显示
eg: select * from student s inner join class c on s.class_id = c.id
2、左连接 left join
以左边数据为主，不满足 on 的条件留在左边，右边数据为null
eg:
select * from student s left join class c on s.class_id = c.id
select s.*,d.dept_no from salaries s left join dept_manager d on s.emp_no = d.emp_no where s.to_date = '9999-01-01'and d.to_date = '9999-01-01';
3、右连接 right join
以右边数据为主，不满足on 的条件留在右边，左边数据为null
eg:
select * from student s right join class c on s.class_id = c.id
select e.last_name,e.first_name,de.dept_no from employees e right join dept_emp de on e.emp_no = de.emp_no