MySQL connection query and subquery

Link query

Link query ： It can also be called cross table query , You need to associate multiple tables to query

Join query classification

According to the age of grammar , Include ：
        SQL92
        SQL99
It is divided according to the connection mode of the table , Include ：
         Internal connection ：
             Equivalent connection
             Non equivalent connection
             Self join
         External connection ：
             The left outer join （ Left connection ）
             Right connection （ The right connection ）
             Full connection

Cartesian product ： When two tables are connected for query , There are no conditions to limit , The number of final query results is the product of the number of records in the two tables .

Example

Display information of each employee , And display the name of the Department

 

  The right thing to do is

  The above query is also called “ Internal connection ”, Query only equal data （ Data with equal connection conditions ）

Internal and external connections

Internal connection ：
         hypothesis A and B Tables to connect , If you use an internal connection , Anyone who A Table and B The table can be queried by matching the records on the table , This is the inner link .AB There is no difference between the two tables , The two tables are equal .

External connection ：
         hypothesis A and B Tables to connect , If you use external connections ,AB One of the two tables is the main table , A watch is a secondary watch , The main query is in the main table   The data of , With the side table , When the data in the secondary table does not match the data in the primary table , The secondary table automatically simulates NULL To match .

External connection classification

The left outer join （ Left connection ）： The table on the left is the main table .
Right connection （ The right connection ）： The table on the right is the main table .    

Example

Show employee information , And display the name of the Department , If a department has no employees , Then the Department must also show

The right connection

Left connection

  The above two queries have the same effect

Subquery

select Nested in statements select sentence , Nested select Statements are subqueries .

Example

Find out who has a higher salary than the average employee , Need to display employee number , Employee name , salary

Realize the idea ：

1. Get an average salary

         select avg(sal) from emp;

      2. Employees who earn more than the average salary

        select empno, ename, sal from emp where sal > (select avg(sal) from emp);