Self join usage of SQL

How to use JOIN Clause , For many different tables , Use public columns , To combine information .

And for the same table , Can also be used JOIN function .

Let's introduce , What is? SELF JOIN, How it works , When do you need it .

In this article, we will discuss what a self join is, how it works, and when you need it in your SQL queries.

When using the table name, use an alias (Aliases), Used to distinguish different contents of the same table .

The grammar is as follows ：

SELECT column_name(s)

FROM table1 T1

JOIN table1 T2

ON condition;

Or not JOIN Clauses can achieve the same effect ：

SELECT column_name(s)

FROM table1 T1, table1 T2

WHERE condition;

Examples are as follows ：

SELECT A.CustomerName AS CustomerName1, B.CustomerName AS CustomerName2, A.City

FROM Customers A

JOIN Customers B

ON A.CustomerID <> B.CustomerID

AND A.City = B.City

ORDER BY A.City;

or

SELECT A.CustomerName AS CustomerName1, B.CustomerName AS CustomerName2, A.City

FROM Customers A, Customers B

WHERE A.CustomerID <> B.CustomerID

AND A.City = B.City

ORDER BY A.City;

above SQL sentence , Will be different in the same city ID All the customers found out .

However, the significance of this example is not very clear , It just introduces the syntax used .

The following example is clearer , The data in the following table ：

Employees surface

Id	FullName	Salary	ManagerId
1	John Smith	10000	3
2	Jane Anderson	12000	3
3	Tom Lanon	15000	4
4	Anne Connor	20000
5	Jeremy York	9000	1

This table shows employee data , Every employee has one ID, And the supervisor's ID. This is a very common organizational structure information .

We now want to display the name information of each employee's supervisor ,SQL The statement is as follows ：

SELECT

    employee.Id,

        employee.FullName,

        employee.ManagerId,

        manager.FullName as ManagerName

FROM Employees employee

JOIN Employees manager

ON employee.ManagerId = manager.Id

The result is ：

Id	FullName	ManagerId	ManagerName
1	John Smith	3	Tom Lanon
2	Jane Anderson	3	Tom Lanon
3	Tom Lanon	4	Anne Connor
5	Jeremy York	1	John Smith

This SQL Inquire about (query) sentence , Display the information of employees with supervisors , And take the supervisor's ID And the name .

And the information in this employee table , Although the information format of each record is the same , But it is classified , There are two roles: employee and supervisor .

So when we execute SELF JOIN when , It's actually the same table , But formally, they are regarded as employees and supervisors .

In terms of expression , We use the alias of the table , Used to distinguish the different roles of the table .

Pay attention to the above employees ID by 4 when , There is no supervisor , If you want to display all employees , Then use LEFT JOIN.

SELECT

    employee.Id,

        employee.FullName,

        employee.ManagerId,

        manager.FullName as ManagerName

FROM Employees employee

LEFT JOIN Employees manager

ON employee.ManagerId = manager.Id

give the result as follows ：

Id	FullName	ManagerId	ManagerName
1	John Smith	3	Tom Lanon
2	Jane Anderson	3	Tom Lanon
3	Tom Lanon	4	Anne Connor
4	Anne Connor
5	Jeremy York	1	John Smith

And the default. JOIN yes INNER JOIN, Use ordinary FROM ... WHERE Clauses can also be substituted .

But if it is LEFT、RIGHT、FULL JOIN, It's useless FROM...WHERE Clause to do .

SLEF JOIN The function of ：

1, Process hierarchical data (Hierarchy)

Data in the same table , A row of data has a hierarchical relationship with other row data . You can be a father son relationship .

adopt LEFT JOIN, To show this relationship .

In addition to the above employees and supervisors , It can also be Human The parent-child relationship in the table ：

Human surface

Id	Name	Age	ParentId
1	Jonathan	5	3
2	Alexandra	7	3
3	Barbara	30

function SQL：

SELECT

    child.Id as ChildId,

        child.FirstName as ChildFirstName,

        child.Age as ChildAge,

        child.ParentId,

        parent.FirstName as ParentFirstName,

        parent.age as ParentAge

FROM Human child

INNER JOIN Human parent

ON child.ParentId = parent.Id

have access to SELF JOIN To the result set of parent-child relationship ：

ChildId	ChildFirstName	ChildAge	ParentId	ParentFirstName	ParentAge
1	Jonathan	5	3	Barbara	30
2	Alexandra	7	3	Barbara	30

There is also the inclusion relationship of categories , Such as the following food classification ：

Category surface

Id	Quantity	Category	ParentCategoryId
1	60	Food
2	50	Fruit	1
3	40	Apple	2
4	20	Granny Smith	3
5	100	Milk	1
6	60	Soy Milk	5
7	40	Cow Milk	5
8	30	Whole Milk	7
9	10	Fat-Free Milk	7

function SQL:

SELECT

    category.Id,

    category.Quantity,

        category.Category,

        category.ParentCategoryId,

        parentcategory.Category as ParentCategory

FROM Category category

JOIN Category parentcategory

ON category.ParentCategoryId = parentcategory.Id

Get the results ：

Id	Quantity	Category	ParentCategoryId	ParentCategory
2	50	Fruit	1	Food
3	40	Apple	2	Fruit
4	20	Granny Smith	3	Apple
5	100	Milk	1	Food
6	60	Soy Milk	5	Milk
7	40	Cow Milk	5	Milk
8	30	Whole Milk	7	Cow Milk
9	10	Fat-Free Milk	7	Cow Milk

So this SELF JOIN The first function of , Is a column as ID, It's a unique identity , Another column identifies the belonging relationship ID, Can be repeated .

The data structure is actually a level diagram of level by level classification .

In execution SELF JOIN when , The correspondence is one-to-one , The number of result sets is less than or equal to the first column ID Count .

2, Show the pairing relationship in the table

Like the following colleagues surface ：

Id	FullName	Age
1	Bart Thompson	43
2	Catherine Anderson	44
3	John Burkin	35
4	Nicole McGregor	29

Suppose we want to generate a table , List all the possibilities of a pair of colleagues , In this way, everyone has the opportunity to talk to each other ：

SELECT

    teammate1.FullName as Teammate1FullName,

    teammate1.Age as Teammate1Age,

        teammate2.FullName as Teammate2FullName,

    teammate2.Age as Teammate2Age

FROM Colleagues teammate1

CROSS JOIN Colleagues teammate2

ON teammate1.FullName <> teammate2.FullName

The result is ：

Teammate1FullName	Teammate1Age	Teammate2FullName	Teammate2Age
Catherine Anderson	44	Bart Thompson	43
John Burkin	35	Bart Thompson	43
Nicole McGregor	29	Bart Thompson	43
Bart Thompson	43	Catherine Anderson	44
John Burkin	35	Catherine Anderson	44
Nicole McGregor	29	Catherine Anderson	44
Bart Thompson	43	John Burkin	35
Catherine Anderson	44	John Burkin	35
Nicole McGregor	29	John Burkin	35
Bart Thompson	43	Nicole McGregor	29
Catherine Anderson	44	Nicole McGregor	29
John Burkin	35	Nicole McGregor	29

The results here , Remove the pairing between yourself and yourself . This relationship is different from the previous one to many , It becomes many to many . This becomes the product relation .

Notice the keywords used here ：CROSS JOIN, Effect and JOIN It's the same , It just means Cartesian product .

such as ：

SELECT * FROM  table1 CROSS JOIN table2;

The result is to table1 and table2 Multiply the records of , Match separately .

3, When combining multiple tables , Multiple operations on the same table JOIN

When combining data between tables , Sometimes multiple information to be added belong to the same table , This is about to JOIN The same table more than once .

The following table ：

Airport Airport Information

AirportId	Country	City
1	USA	New York
2	Canada	Toronto
3	Germany	Frankfurt
4	France	Paris
5	Italy	Rome

Flight flight

FlightId	AirplaneId	StartTimestamp	EndTimestamp	StartAirportId	EndAirportId
2	555877	2020-01-14 13:00:00	2020-01-14 15:00:00	3	4
3	222536	2020-02-04 01:00:00	2020-02-04 16:00:00	1	5
4	111745	2020-02-15 09:00:00	2020-02-15 12:00:00	5	4
5	777524	2020-02-24 03:00:00	2020-02-24 19:00:00	4	2
6	888521	2020-03-25 10:00:00	2020-03-25 12:00:00	2	1
7	444937	2020-04-01 00:00:00	2020-04-01 17:00:00	3	1
243	111654	2020-01-01 02:00:00	2020-01-01 04:00:00	1	2

To display the start and end information of the flight , Will be JOIN The airport watch twice ：

SELECT

        flight.FlightId,

        flight.AirplaneId,

        flight.StartAirportId,

        startairport.Country as StartAirportCountry,

        startairport.City as StartAirportCity,

        flight.EndAirportId,

        endairport.Country as EndAirportCountry,

        endairport.City as EndAirportCity

FROM Flight flight

JOIN Airport startairport

ON flight.StartAirportId = startairport.AirportId

JOIN Airport endairport

ON flight.EndAirportId = endairport.AirportId

Get the results ：

FlightId	AirplaneId	StartAirportId	StartAirportCountry	StartAirportCity	EndAirportId	EndAirportCountry	EndAirportCity
1	111654	1	USA	New York	2	Canada	Toronto
2	555877	3	Germany	Frankfurt	4	France	Paris
3	222536	1	USA	New York	5	Italy	Rome
4	111745	5	Italy	Rome	4	France	Paris
5	777524	4	France	Paris	2	Canada	Toronto
6	888521	2	Canada	Toronto	1	USA	New York
7	444937	3	Germany	Frankfurt	1	USA	New York

Reference resources ：

What Is a Self Join in SQL? An Explanation With Seven Examples | LearnSQL.com

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