Inner join and outer join of MySQL tables

Internal connection

Internal connection is actually the use of where Clause to filter the Cartesian product formed by the two tables , The queries we learned earlier are inner joins , It is also the most used connection query in the development process .

grammar ：

select  Field  from  surface 1 inner join  surface 2 on  Connection condition  and  Other conditions ;

Case study ： Show SMITH Your name and department name

1. It was written in the original way

2. Standard inner join writing

External connection

The outer connection is divided into left outer connection and right outer connection

The left outer join

If the union query , The table on the left shows that it is a left outer join .

grammar ：

select  Field name  from  Table name 1 left join  Table name 2 on  Connection condition 

example ：

Build table

Check the grades of all the students , If the student has no grades , Also display the student's personal information

Right connection

If the union query , The table on the right shows that we call it right outer join .

grammar ：

select  Field  from  Table name 1 right join  Table name 2 on  Connection condition ;

example

Yes stu Table and exam Table union query , Show all your grades , Even if there are no students corresponding to this grade , Show it, too