MySQL - multi table query and case explanation

Catalog

Overview of multi table query

Internal connection

External connection

1). The left outer join

2). Right connection

Self join

The joint query

Subquery

Scalar subquery

Column query

Line sub query

Table sub query

Multi table query exercise

Implicit inner join

Explicit inner connection

duplicate removal distinct

The left outer join

between ... and ...

Three table query

function AVG

Scalar subquery

Named references

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Overview of multi table query

classification

Internal connection ： It is equivalent to a query A、B Intersection part data

External connection ： There are two types of external connections , Namely ： The left outer join and Right connection

The left outer join ： Query all data in the left table , And the intersection of two tables

Right connection ： Query all data in the right table , And the intersection of two tables

Self join ： Query the connection between the current table and itself , Self join must use table alias

Data preparation

drop table dept;
drop table emp;

--  establish dept surface , And insert data 
create table dept(
id int auto_increment comment 'ID' primary key,
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 ');

--  establish emp surface , And insert data 
create table emp(
id int auto_increment comment 'ID' primary key,
name varchar(50) not null comment ' full name ',
age int comment ' Age ',
job varchar(20) comment ' Position ',
salary int comment ' Salary ',
entrydate date comment ' Entry time ',
managerid int comment ' Direct leadership ID',
dept_id int comment ' department ID'
)comment ' The employee table ';

--  Add foreign keys 
alter table emp add constraint fk_emp_dept_id foreign key (dept_id) references
dept(id);
INSERT INTO emp (id, name, age, job,salary, entrydate, managerid, dept_id)
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, ' chang yuchun ', 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, ' Crane pen Weng ', 19, ' staff member ',3750, '2007-05-09', 10,2),
(13, ' Fang Dongbai ', 19, ' staff member ',5500, '2009-02-12', 10,2),
(14, ' Zhang Sanfeng ', 88, ' Sales Director ',14000, '2004-10-12', 1,4),
(15, ' Yu Lianzhou ', 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

There are two types of external connections , Namely ： The left outer join and Right connection

1). Implicit inner join

SELECT Field list FROM surface 1 , surface 2 WHERE Conditions ... ;

2). Explicit inner connection

SELECT Field list FROM surface 1 [ INNER ] JOIN surface 2 ON Connection condition ... ;

A. Check the name of each employee , And the name of the associated department ( Implicit inner connection implementation )

Table structure : emp , dept

Connection condition : emp.dept_id = dept.id

select emp.name , dept.name from emp , dept where emp.dept_id = dept.id ;
--  Alias each table , simplify SQL To write 
select e.name,d.name from emp e , dept d where e.dept_id = d.id;

B. Check the name of each employee , And the name of the associated department ( Explicit inner join implementation ) --- INNER JOIN ...

ON ...

Table structure : emp , dept

Connection condition : emp.dept_id = dept.id

select e.name, d.name from emp e inner join dept d on e.dept_id = d.id;
--  Alias each table , simplify SQL To write 
select e.name, d.name from emp e join dept d on e.dept_id = d.id;

Be careful ： Once the table is aliased , You can no longer use the table name to specify the corresponding field , At this point, only aliases can be used to specify fields

External connection

There are two types of external connections , Namely ： The left outer join and Right connection .

1). The left outer join

SELECT Field list FROM surface 1 LEFT [ OUTER ] JOIN surface 2 ON Conditions ... ;

The left outer join is equivalent to a query table 1( The left table ) All data for , Of course, it also includes tables 1 And table 2 The data of the intersection part

2). Right connection

SELECT Field list FROM surface 1 RIGHT [ OUTER ] JOIN surface 2 ON Conditions ... ;

Right outer join is equivalent to query table 2( Right table ) All data for , Of course, it also includes tables 1 And table 2 The data of the intersection part .

A. Inquire about emp All data of table , And the corresponding department information

Because... Is mentioned in the demand , To query emp All data for , Therefore, internal connection query is not allowed , You need to consider using external join queries .

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

select e.*, d.name from emp e left join dept d on e.dept_id = d.id;

B. Inquire about dept All data of table , And corresponding employee information ( Right connection )

Because... Is mentioned in the demand , To query dept All data of table , Therefore, internal connection query is not allowed , You need to consider using external connections to check

Inquiry .

select d.*, e.* from emp e right outer join dept d on e.dept_id = d.id;

select d.*, e.* from dept d left outer join emp e on e.dept_id = d.id;

matters needing attention ：

The left outer connection and the right outer connection can be replaced with each other , You only need to adjust when connecting queries SQL in , The order of table structure

Just order it . And when we use it in daily development , More inclined to the left outer connection

Self join

Self connect query , seeing the name of a thing one thinks of its function , Is to connect yourself , That is to connect and query a table many times .

SELECT Field list FROM surface A Alias A JOIN surface A Alias B ON Conditions ... ;

For self join queries , It can be internal connection query , It can also be an external connection query

A. Query the names of employees and their leaders （ Internal connection ）

select a.name , b.name from emp a , emp b where a.managerid = b.id;

B. Query all employees emp And the names of their leaders emp , If the employee has no leader , You also need to find out （ External connection ）

select a.name ' staff ', b.name ' Leader ' from emp a left join emp b on a.managerid = b.id;

matters needing attention :

In self join query , You must alias the table , Otherwise, we don't know the specified conditions 、 Fields returned , Which table field is it .

The joint query

about union Inquire about , Is to combine the results of multiple queries , Form a new query result set

SELECT Field list FROM surface A ...

UNION [ ALL ]

SELECT Field list FROM surface B ....;

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

A. Lower salary than 5000 The employees' , and Older than 50 All the employees aged 18 are found out

select * from emp where salary < 5000
union all
select * from emp where age > 50;

union The joint query , The query results will be de reprocessed

select * from emp where salary < 5000
union 
select * from emp where age > 50;

Subquery

SELECT * FROM t1 WHERE column1 = ( SELECT column1 FROM t2 );

The statement outside the subquery can be INSERT / UPDATE / DELETE / SELECT Any one of

According to the sub query results , It is divided into ：

A. Scalar subquery （ The subquery result is a single value ）

B. Column query ( The subquery result is a column )

C. Line sub query ( The sub query result is one line )

D. Table sub query ( The sub query result is multi row and multi column )

According to the sub query location , It is divided into ：

A. WHERE after

B. FROM after

C. SELECT after

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Scalar subquery

Common operators ：= <> > >= < <=

Inquire about " The sales department " All employee information for

select * from emp where dept_id = (select id from dept where name = ' The sales department '); 

The query in " Fang Dongbai " Employee information after employment

select * from emp where entrydate > (select entrydate from emp where name = ' Fang Dongbai ');

Column query

Common operators ：

Inquire about " The sales department " and " The Marketing Department " All employee information for

select * from emp where dept_id in (select id from dept where name = ' The sales department ' or name = ' City 
 Field department ');

Query ratio Finance Department Information about employees whose wages are high for everyone

select * from emp where salary > all ( select salary from emp where dept_id = (select id 
from dept where name = ' Finance Department ') );

Query the information of employees whose salary is higher than that of any one in the R & D department

select * from emp where salary > any ( select salary from emp where dept_id = (select id from dept where name = ' R & D department ') );

Line sub query

Common operators ：= 、<> 、IN 、NOT IN

Query and " zhang wuji " The salary and the same employee information as the direct leader

select * from emp where (salary,managerid) = (select salary, managerid from emp where name = ' zhang wuji ');

Table sub query

Common operators ：IN

A. Query and " Deer stick guest " , " Song Yuanqiao " Employee information with the same position and salary

select * from emp where (job,salary) in ( select job, salary from emp where name = ' Deer stick guest ' or name = ' Song Yuanqiao ' );

B. The date of employment inquiry is "2006-01-01" Later employee information , And its department information

select e.*, d.* from (select * from emp where entrydate > '2006-01-01') e left join dept d on e.dept_id = d.id ;

Multi table query exercise

Data environment preparation :

create table salgrade(
grade int,
losal int,
hisal int
) 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 (8,25001,30000);

Implicit inner join

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

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

Explicit inner connection

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

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

duplicate removal distinct

3). Query the Department with employees ID、 Department name

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

The left outer join

4). Query all ages greater than 40 Year old employees , And the name of the Department to which it belongs ; If the employee is not assigned a department , Also need to show

Come on ( External connection )

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

between ... and ...

5). Query the salary grade of all employees

--  Mode one 
select e.* , s.grade , s.losal, s.hisal from emp e , salgrade s where e.salary >=
s.losal and e.salary <= s.hisal;
--  Mode two 
select e.* , s.grade , s.losal, s.hisal from emp e , salgrade s where e.salary
between s.losal and s.hisal;

Three table query

6). Inquire about " R & D department " Information and of all employees Pay scale

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

function AVG

7). Inquire about " R & D department " Average salary of employees

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

Scalar subquery

8). Check the wage ratio " extinction " High employee information

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

9). Query employee information with higher than average salary

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

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

select * from emp e2 where e2.salary < ( select avg(e1.salary) from emp e1 where e1.dept_id 
= e2.dept_id );

11). Look up all the Department Information , And count the number of employees in the Department （select ... from Sub queries can also be used between ）

select d.id, d.name , ( select count(*) from emp e where e.dept_id = d.id ) ' The number of ' from dept d;

Named references

12). Check the course selection of all students , Show the student's name , Student number , Course name

select s.name , s.no , c.name from student s , student_course sc , course c where s.id = 
sc.studentid and sc.courseid = c.id ;