Over 10 table queries - even the operating table

# Built table 
Create Table Department ( 
ID int, 
name VARCHAR ( 20 is ) 
); 

Create Table Employee ( 
ID int Primary Key AUTO_INCREMENT, 
name VARCHAR ( 20 is ), 
Sex enum ( ' MALE ' , ' FEMALE ' ) Not null default ' MALE ' , 
Age int, 
the dep_id int 
); 

# insert data 
iNSERT INTO Department values 
( 200 is, ' technical ' ), 
( 201, ' HR '), 
( 202, ' sales ' ), 
( 203, ' operation ' ); 

INSERT INTO Employee (name, Sex, Age, the dep_id) values 
( ' Egon ' , ' MALE ' , 18,200 ), 
( ' Alex ' , ' FEMALE ' , 48,201 ), 
( ' wupeiqi ' , ' MALE ' , 38,201 ), 
( ' yuanhao ' , 'female', 28,202 ), 
( ' liwenzhou ' , ' MALE ' , 18,200 ), 
( ' jingliyang ' , ' FEMALE ' , 18,204 ) 
;

 

En:

select * from employee inner join department on employee.dep_id=department.id;

 

Record retention, including the connection of the left table on the basis of: Left connection

select * from employee left join department on employee.dep_id=department.id;

 

Right Connections:

select * from employee right join department on employee.dep_id=department.id;

 

Full outer connector: reserved table do not correspond to the right and left inner connection on the basis of the recording

select * from employee left join department on employee.dep_id=department.id

union

select * from employee right join department on employee.dep_id=department.id;

 

# Query sector average age greater than 30 years old

 

select department.name,avg(age) from employee inner join department on employee.dep_id=department.id
group by department.name
having avg(age)>30;

 

 

SELECT statement to define the order of keywords

The execution order of the SELECT statement keywords

FROM execution

ON execution

Implementation of external lines