Query method of database multi table link

Tips ： Used in this period SQL sentence ,MySQL database ,Navicat Database management tools

 

Catalog

Preface

One 、 The tables you need to use

① Student list (student)：

② The curriculum (course):

③ Class table (class):

Two 、 Non equivalent connection

3、 ... and 、 Equivalent connection

  1. Equivalent connection query method

  2. Internal connection （inner join）

  3. Left outer connection query (left join)

  4. Right outer connection query (right join)

  5. All external connection query （union）

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Preface

First , We need to know why database tables are multiple tables , Because designing a table will cause data redundancy , Cause a waste of database space , However, sometimes we need to integrate data from multiple tables and query them , At this time, you need to query by associating the primary and foreign key relationships between tables .

This article will divide according to the connection mode of the table , Non equivalent connection , Equivalent connection （ Internal connection , The left outer join （ Left connection ）, Right connection （ The right connection ）, Full outer join ） Two parts are explained .

One 、 The tables you need to use

① Student list (student)：

CREATE TABLE `student` (
  `Sid` int(11) NOT NULL AUTO_INCREMENT,
  `Sname` varchar(10) DEFAULT NULL,
  `birthday` datetime DEFAULT NULL,
  `Ssex` varchar(10) DEFAULT NULL,
  `classid` int(11) DEFAULT NULL,
  PRIMARY KEY (`Sid`)
)

  

② The curriculum (course):

CREATE TABLE `course` (
  `Cid` int(11) NOT NULL AUTO_INCREMENT,
  `Cname` varchar(10) DEFAULT NULL,
  `Tid` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`Cid`)
) 

③ Class table (class):

CREATE TABLE `class` (
  `classid` int(11) NOT NULL AUTO_INCREMENT,
  `classname` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`classid`)
) 

  

Two 、 Non equivalent connection

         Definition ：

        The non equivalent connection is the Cartesian product , It refers to any combination of all elements in two relationships . In general , Non equivalent queries are useless , Because if the amount of data is too large , The table formed by Cartesian product will take up a lot of space .

  Add : Cartesian product is also called direct product , Express XY, such as A The data in the table are m That's ok ,b The data in the table are n That's ok , that A and B Do Cartesian product , The result is mn That's ok .

         grammar ：

SELECT * from  surface 1 CROSS JOIN  surface 2;

         give an example ：（ Student schedule and Curriculum ） 

SELECT * from student CROSS JOIN course;

          For example, the first article will show all the courses of Zhao Lei .

3、 ... and 、 Equivalent connection

  1. Equivalent connection query method

         Definition ：

                 Use the equal sign operator in the connection condition to compare the values of the connected fields , The virtual table generated by it will show all the columns of the connected table , Include duplicate Columns . 

         grammar ：        

select * from  surface 1, surface 2 where  surface 1. Field = surface 2. Field 2;

         give an example : ( Student list and class list )

SELECT * FROM student,class WHERE student.classid=class.classid;

  2. Internal connection （inner join）

        grammar ：

select * from  surface 1 inner join  surface 2 on  Conditions ;

 

   give an example : ( Student list and class list )

select * from student inner join class on student.classid=class.classid;

​​​​​​​

  3. Left outer connection query (left join)

        grammar ： 

select * from  surface 1 left join  surface 2 on  surface 1. Field = surface 2. Field ;

   give an example : ( Student list and class list )

  The student table will be used as the main table for query , Zhang San will show up without class .

select * from student LEFT join class on student.classid=class.classid;

 ​​​​​​​​​​​​​​

  4. Right outer connection query (right join)

         grammar ： 

select * from  surface 1 right join  surface 2 on  surface 1. Field = surface 2. Field ;

   give an example : ( Student list and class list )

  The class table will be displayed as the main table , As shown in the figure, no one in class 4 will also show .

select * from student right join class on student.classid=class.classid;

  5. All external connection query （union）

         grammar ：  

select * from  surface 1 left join  surface 2 on  surface 1. Field = surface 2. Field 
union
select * from  surface 1 right join  surface 2 on  surface 1. Field = surface 2. Field ;

 

  give an example : ( Student list and class

select * from student LEFT join class on student.classid=class.classid
UNION
select * from student right join class on student.classid=class.classid;

 

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