Mysql internal and external connections

1. Inner join

• Inner joins actually usewhereThe clause filters the Cartesian product formed by the two tables, weThe queries learned earlier are all inner joins, and they are also the most used join queries in the development process.
• Syntax:

select fields from table 1 inner join table 2 on join conditions and other conditions;

         

• Case: Display SMITH's name and department name

①Front writing

• Where used here is to find information that satisfies the following conditions in all the results after the Cartesian product is performed before

select ename, dname from EMP, DEPT where EMP.deptno=DEPT.deptno and ename='SMITH';

②Standard inner join (recommended, elegant)

select ename , dname from EMP inner join DEPT on EMP.deptno = DEPT.deptno where ename = 'SMITH';

      

2. Outer join

(1) Left outer join

• If the joint query, the table on the left is completely displayed, we say it is a left outer join.
• Take the information in the left table and go to the right table to filter, and all that are satisfied will be displayed, even if there are multiple

select field name from table name 1 left join table name 2 on join condition;

         

Case: Query the grades of all students, if the student has no grades, also display the student's personal information

• When the left table and the right table do not match, the data of the left table will also be displayedspan>

      

(2) Right outer join

• If the joint query, the table on the right is completely displayed, we say it is a right outer join.

select field from table name 1 right join table name 2 on join condition;

      

Case: Jointly query the stu table and the exam table, and display all the grades, even if there is no student corresponding to this grade, it should be displayed

         

(3)Exercise : List department names and employee information in these departments, and list departments without employees

      

      

3. Summary

• When querying, there is a high probability that multiple tables will be involved, and it is often necessary to "merge" multiple tables into one table. The essence of all queries is to convert them into one table query!!
• Self-join, from (sub-query), and internal and external joins are essentially answers to how to complete the merging of multiple tables!!
• Inner join: keep the common connection conditions, left outer join: keep all the information of the left table, right outer join: keep all the information of the right table.