Multiple table connections

Table connection introduction

With the development of business , The relationship between tables will be closer and closer , Table connected sql There will be more and more operations , Classification of table connections ：

• Internal connection
• External connection
• Cross connect
• Natural join
• Self join

test surface ：

test_age surface ：

Internal connection

Use join on and where Statement connection . Return the data of both parties

SELECT t.kemu from test t join test_age ta where t.name = ta.name

External connection

The left outer join

left join on ： The left outer join , Return all data of the left table , If there is data in the right table, it returns , If it does not exist, a null value is returned .

select t.kemu,e.age from test t LEFT join test_age e on t.name = e.name 

Right connection

right join on ： Right connection , Return all data of the right table , If there is data in the right table, it returns , If it does not exist, a null value is returned .

select t.kemu,e.age from test t right join test_age e on t.name = e.name 

Cross connect

Universal Cartesian product ,cross join

Natural join

USING ON Simplify connection conditions , The connection conditions must be equivalent , Field names must be the same , Same type .

select t.kemu,e.age from test t  join test_age e using(name)