Multi table (Association) query of SQL query data

Data sheet ：

Association queries are mainly divided into ：（inner join） Cross correlation 、（left join） Left Association 、（right join） On the right

（inner join） Cross correlation ：

1. Definition ：（inner join） Cross association only returns rows with equal fields in two tables
2. grammar ：select * from surface 1

inner join surface 2

on surface 1. Field number = surface 2. Field number

For example, I want to check the cross correlation between student table and grade table , First of all, find the fields with equal connection

The only field they share is the student number

Then write it according to the above grammar ：

1. Look for the , Output table is only 5 Information of students , But our student list is shared 10 Of a student , Its definition is to return only rows with equal fields in two tables ,

There are only 5 Student number of students , Relative to the student table, the only connection can be 5 Student information

（left join） Left Association ：

1. Definition ：（left join） The left Association returns all records in the left table and records with the same join fields in the right table
2. grammar ：select * from surface 1

left join surface 2

on surface 1. Field number = surface 2. Field number

Then check the left correlation between the student table and the grade sheet. First type it with reference to the grammar , The effect is as follows ：

Left association is based on the left table , The data in the right table shows , No data shows null

（right join） On the right ：

1. Definition ：（right join） Right association returns all records in the right table and records with the same join fields in the left table
2. grammar ：select * from surface 1

right join surface 2

on surface 1. Field number = surface 2. Field number

Right association is just opposite to left Association