Mysql 左連接、右連接，內連接區別

1、內連接 inner join  

JOIN 相當於INNER JOIN內連接，不滿足on 的條件過濾掉，不顯示

eg:   select * from student s inner join class c on s.class_id = c.id

 

2、左連接  left join

以左邊數據為主，不滿足 on 的條件留在左邊，右邊數據為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、右連接  right join

以右邊數據為主，不滿足on 的條件留在右邊，左邊數據為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