MySQL - multi table query

MySQL---- Multi-table query

• • Multi table relation
  • • The connection between tables ：
    • One to many ( For one more )
    • Many to many
    • one-on-one
  • Multi-table query
  • Classification of multi table query
  • Data preparation
  • Internal connection
  • External connection
  • Self join
  • The joint query
  • Subquery
  • • Scalar subquery
    • Column query
    • Line sub query
    • Table sub query
  • Multi table query case

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Multi table relation

When designing the database table structure , According to the business requirements and the relationship between business modules , Analysis and design table structure , Because businesses are interrelated , Therefore, there are also various connections between various table structures

The connection between tables ：

1. One to many ( For one more )
2. Many to many
3. one-on-one

One to many ( For one more )

for example , One employee corresponds to one department , A department can correspond to multiple employees

Usually create a foreign key on the side of more than one , Point to the side of one
Employees and departments , Set foreign keys on the employee table , Point to the Department table

Many to many

for example , A student can take more than one course , A course can be taken by more than one student
A third table is usually created , Contains at least two foreign keys , Point to the primary keys of the two tables

one-on-one

for example , The relationship between users and their academic information , One person only corresponds to one piece of education information
You can add foreign keys on either side , Associate the primary key of the other party , And set the foreign key to be unique (unique)

notes ： It can be put in a table , But split it , Put basic information in a table , Put the details in another table , It can improve the operation efficiency

Multi-table query

summary ：
Query data from multiple tables
The cartesian product ：
Cartesian product is two sets ( Two tables ) The result of pairwise combination of each data in
Cartesian product will be generated in multi table query , To eliminate the Cartesian product by adding conditions

dept surface ：

emp surface ：

The query produces the result of Cartesian product ：

select * from emp, dept ;

Eliminate Cartesian product ( Adding conditions )：

select * from emp, dept where emp.dept_id=dept.id;

Classification of multi table query

1. Link query ：

 Internal connection ：
	 It is equivalent to a query AB The intersection of 
 External connection ：
		 The left outer join ：
			 Inquire about A All data for , Splice at the same time B Corresponding data 
		 Right connection ：
			 Inquire about B All data for , Splice at the same time A The corresponding data in 
 Self join ：
	 Table and its own connection query 
	 Self join must alias the table 

2. Subquery

Data preparation

Departmental table ：

create table dept (
    id int auto_increment primary key comment 'id',
    name varchar(50) not null comment ' Department name '
) comment ' Departmental table ';

insert into dept (id, name)
values (1, ' R & D department '),
       (2, ' The Marketing Department '),
       (3, ' Finance Department '),
       (4, ' The sales department '),
       (5, ' General manager office '),
       (6, ' The personnel department ');

The employee table ：

create table emp(
    id int auto_increment primary key ,
    name varchar(50) not null ,
    age int,
    job varchar(20) comment ' Position ',
    salary int ,
    entrydate date comment ' Entry time ',
    managerid int comment ' Direct leadership id',
    dept_id int comment ' Department id'
) comment ' The employee table ';

insert into emp
values ( 1, ' Jin yong ', 66, ' President ', 20000, '2000-01-01', null, 5 ),
       ( 2, ' zhang wuji ', 20, ' project manager ', 12500, '2005-12-05', 1, 1 ),
       ( 3, ' Yang Xiao ', 33, ' Development ', 8400, '2000-11-03', 2, 1 ),
       ( 4, ' Xiangr ', 48, ' Development ', 11000, '2002-02-05', 2, 1 ),
       ( 5, ' Chen Yucun ', 43, ' Development ', 10500, '2004-09-07', 3, 1 ),
       ( 6, ' Small zhao ', 19, ' Programmers encourage teachers to ', 6600, '2004-10-12', 2, 1 ),
       ( 7, ' extinction ', 60, ' Chief financial officer ', 8500, '2002-09-12', 1, 3 ),
       ( 8, ' Zhou Zhiruo ', 19, ' accounting ', 48000, '2006-06-02', 7, 3 ),
       ( 9, ' Ding Minjun ', 23, ' Cashier ', 5250, '2009-05-13', 7, 3 ),
       ( 10, ' Zhao Min ', 20, ' Director of marketing department ', 12500, '2004-10-12', 1, 2 ),
       ( 11, ' Deer stick guest ', 56, ' staff member ', 3750, '2006-10-03', 10, 2 ),
       ( 12, ' Hebiwen ', 19, ' staff member ', 3750, '2007-05-09', 10, 2 ),
       ( 13, ' Oriental White ', 19, ' staff member ', 5500, '2009-02-12', 10, 2 ),
       ( 14, ' Zhang Sanfeng ', 88, ' Sales Director ', 14000, '2004-10-12', 1, 4 ),
       ( 15, ' Yuliangzhou ', 38, ' sales ', 4600, '2004-10-12', 14, 4 ),
       ( 16, ' Song Yuanqiao ', 40, ' sales ', 4600, '2004-10-12', 14, 4 ),
       ( 17, ' Chen Youlang ', 42, null, 2000, '2011-10-12', 1, null );

Internal connection

grammar ：

#  Implicit inner join 
select  Field list  from  surface 1, surface 2 where  Conditions ;
#  Display inner connection 
select  Field list  from  surface 1 [inner] join  surface 2 on  Connection condition ;

The inner join query is the part of the intersection of two tables

#  Query the name of each employee and the name of the associated department 
select emp.name, dept.name from emp, dept where emp.dept_id=dept.id;
select emp.name, dept.name from emp inner join dept on emp.dept_id = dept.id;

External connection

grammar ：

#  The left outer join 
select  Field list  from  surface 1 left [outer] join  surface 2 on  Conditions ;
#  Right connection 
select  Field list  from  surface 1 right [outer] join  surface 2 on  Conditions ;

The left outer join is equivalent to a query table 1 All of the data contained in the table 1 And table 2 Some data of intersection
Right outer join is equivalent to query table 2 All of the data contained in the table 1 And table 2 The data of the intersection part

#  Inquire about emp All data of table , And corresponding department information ( Left )
select emp.*, dept.* from emp left outer join dept on emp.dept_id = dept.id;
#  Inquire about dept All data of table , And employee information for ( Right )
select dept.*, emp.* from emp right outer join dept on emp.dept_id = dept.id;

The left outer connection and the right outer connection can be transformed into each other

Self join

grammar ：

select  Field list  from  surface a  Alias a join  surface a  Alias b on  Conditions ;

The self link query can be an internal connection query or an external connection query

#  Query the names of employees and their leaders 
#  Self join can be viewed as two identical tables for join query 
select a.name, b.name from emp a join emp b on a.managerid=b.id;

The joint query

union、union all
For joint query, the results of multiple queries are merged , Form a new query result set
grammar ：

select  Field list  from  surface a
union [all]
select  Field list  from  surface b


#  Lower salary than 5000 Of employees and older than 50 Of employees 
select * from emp where salary>5000
union all
select * from emp where age>50;


#  No, all If the condition is met repeatedly, it only occurs once 
#  Lower salary than 5000 Of employees and older than 50 Of employees 
select * from emp where salary>5000
union
select * from emp where age>50;

The number of columns in multiple tables of joint query must be consistent , Field types should also be consistent
union all All the data will be merged directly ,union The merged data will be de duplicated

Subquery

Concept ：SQL Nested in statement select Statement is a nested query , Also known as subquery
select * from surface 1 where Field =(select Field from surface 2);
Statements outside the subquery can be insert、update、delete、select One of them
According to the structure of sub query , It is divided into ：

 Scalar subquery ： The result of the subquery is a single value 
 Column query ： The result of the subquery is a column 
 Line sub query ： The result of the subquery is one row 
 Table sub query ： The result of subquery is multi row and multi column 

According to the location of subquery , It is divided into ：

where after 
from after 
select after 

Scalar subquery

The result returned by the subquery is a single value ( Numbers 、 character string 、 Date, etc. ), The simplest form , This seed query is called a scalar subquery
Common symbols ：=、<>、>、>=、<、<=

#  According to the sales department id Query employee information 
#  First, query separately 
#  Query the sales department id
select id from dept where name=' The sales department '; #id by 4
#  Query the information of employees in the sales department 
select * from emp where dept_id=4;
#  Merge into one query 
select * from emp where dept_id=(select dept.id from dept where dept.name=' The sales department ' );

Column query

The result of the subquery is a column ( It can be multiple lines ) Of , This sub query is a column sub query
Common operators ：

#  Column query 
#  Query all employee information of sales department and marketing department 
#  Check the sales and marketing department id
select id from dept where name=' The sales department ' or name=' The Marketing Department '; #id by 2 4
#  Query all employees in both departments 
select * from emp where dept_id in (2,4);
#  Merge 
select * from emp where dept_id in (select id from dept where name=' The sales department ' or name=' The Marketing Department ');

Line sub query

The result returned by the subquery is a row ( It can be multiple columns ), This sub query is a row sub query
Common operators ：=、<>、in、not in

#  Query the same employee information as Zhang Wuji's salary and direct leaders 
#  Inquire about Zhang Wuji's salary and direct leaders 
select salary, managerid from emp where name=' zhang wuji ';
#  Query the same employee information as Zhang Wuji's salary and direct leaders 
select * from emp where (salary,managerid)=(select salary, managerid from emp where name=' zhang wuji ');

Table sub query

The result of subquery is multi row and multi column. This kind of query is table subquery
Common operators ：in

#  Query the employee information that is the same as the position and salary of Lu zhanke and song Yuanqiao 
select * from emp where (job, salary) in ( select job, salary from emp where name in (' Deer stick guest ', ' Song Yuanqiao '));

The sub tables of the table sub query are used as temporary tables

#  The date of employment inquiry is ’2006-01-01‘ After that, employee information and department information 
#  First, check the entry and exit positions ’2006-01-01‘ After that, all the information of the employees 
#  Connect to the left of the Department table 
select e.*, dept.* from (select * from emp where entrydate>'2006-01-01') e left outer join dept on e.dept_id=dept.id;

Multi table query case

Data preparation ：

create table salgrade (
    grade int,
    losal int comment ' The minimum limit of this salary grade ',
    hisal int comment ' The highest limit '
) comment ' Salary scale ';
insert into salgrade values (1,0,3000);
insert into salgrade values (2,3001,5000);
insert into salgrade values (3,5001,8000);
insert into salgrade values (4,8001,10000);
insert into salgrade values (5,10001,15000);
insert into salgrade values (6,15001,20000);
insert into salgrade values (7,20001,25000);
insert into salgrade values (8,025001,30000);

1. Check the name of the employee , Age , Position , Department information ( Implicit inner join )

select e.name, e.age, e.job, d.* 
from emp e, dept d 
where e.dept_id=d.id;

2. The query age is less than 30 The name of the employee 、 Age 、 Position 、 Department information ( Display inner connection )

select e.name,e.age,e.job,d.*
from emp e
inner join dept d on e.dept_id = d.id
where e.age<30;

3. Query the Department with employees id, Department name

select distinct d.id,d.name
from emp e, dept d
where d.id=e.dept_id;

4. Query all ages greater than 40 The employees' , And the name of the Department to which it belongs , If the employee has no assigned department, it should also display

select e.*,d.name
from emp e
left outer join dept d on e.dept_id = d.id
where e.age>40;

5. Query the salary grade of all employees

select e.*,s.grade
from emp e, salgrade s
where e.salary between s.losal and s.hisal;

6. Query the information of all employees in the R & D department, i.e. salary grade

select e.*,s.grade
from emp e,dept d,salgrade s
where (e.dept_id=d.id) and (d.name=' R & D department ') and (e.salary between s.losal and s.hisal);

7. Query the average salary of employees in the R & D department

select avg(e.salary)
from emp e, dept d
where e.dept_id=d.id and d.name=' R & D department ';

8. Query the information of employees whose wages are higher than extinction

select *
from emp
where emp.salary > (
                      select e.salary
                      from emp e
                      where e.name=' extinction '
                      );

9. Query employee information with higher than average salary

select *
from emp
where salary> (
    select avg(e.salary)
    from emp e
    );

10. Query the information of employees whose salary is lower than the average salary of the Department

select *
from emp
where emp.salary<(
    select avg(salary)
    from emp e
    where e.dept_id=emp.dept_id
    );

11. Check all department information , And count the number of employees in the Department

select d.*, (
    select count(*)
    from emp
    where emp.dept_id=d.id
    )
from dept d;