SQL Self Join

Updated March 27, 2023

Introduction to SQL Self Join

A self-join is a Structured Query Language (SQL) statement in which a table is joined with itself, an inner join is performed on a single table with itself, particularly in cases when comparisons have to be made between the records of the same table to determine a relationship or in the cases when the table has a FOREIGN KEY which references its own PRIMARY

Syntax

The generic syntax for working with SQL SELF Join is as follows :

SELECT t1.column_name, t2.column_name
FROM table_name1 t1, table_name1 t2
WHERE t1.common_filed = t2.common_field;

Parameters in SQL Self Join

The different parameters used in the syntax are :

• SELECT t1.column_name, t2.column_name: It is used to select the required data from the database. Here, t1.column_name is the column from the table’s instance t1. t2.column_name is the column from the table’s instance t2.
• FROM table_name1 t1, table_name1 t2: It is used to specify the source from which data has to be fetched. Since in self-join, we perform an inner join on a single table. We create two instances of the table as t1 and t2.
• WHERE t1.common_filed = t2.common_field: It is used to specify the conditions to filter records.In self join we will be mentioning the condition on which the two instances of the table, namely t1 and t2 will join. This is the common field on which tables will join.

Of the above-mentioned parameters, all the parameters are mandatory. You may use GROUP BY, ORDER BY and HAVING clauses based on your requirement.

How does SQL Self Join work?

Self-join is mostly used in cases where we have :

• Hierarchical Relationships
• Sequential Relationships

Self join works by joining a table with itself on a defined condition. For example, let’s assume that we have a group of students and they have a best friend relationship with some other student in the same table and we want to know the name of the student and his friend.

Id	Student_name	Friend_id
1	JAY	3
2	RAY	1
3	MAY	2

• Now, in order to get the name of each student along with his friend, we can perform a self-join which will join the table something like this on the condition that friend id is equal to student_id.

• The resultant table will look something like this :

Id	Student_name	Friend
1	JAY	MAY
2	RAY	JAY
3	MAY	RAY

• Going ahead we will be discussing the above-mentioned self join in great detail. In order to demonstrate and explain the Self join in SQL effectively, we will be using the following table. This table is made for an e-commerce website. The table contains customer id, names, city, and the country to which they belong. The schema for the above mentioned “customers” table is :
• Number of Records: 15

Customers

ID(primary key)

Customer

City

Country

• Let’s have a look at the records in the customer’s table. So that later, we can understand how self join is helpful:

ID	Customer	City	Country	Items_purchased	Amount_paid
1	Peter King	Manchester	England	Books	120
2	Priya Krishna	New Delhi	India	pen	50
3	Jim Halpert	Manchester	England	pencil	43
4	Michael Scott	New York	USA	Books	250
5	Harvey Spector	Birmingham	England	pencil	100
6	Deepa Kamat	Mumbai	India	Books	370
7	Anita Desai	London	England	pencil	50
8	Rachel Zane	Michigan	USA	pen	70
9	Pretoria John	Canberra	Australia	pen	190
10	John L	Budapest	Hungry	Books	540
11	Justin Green	Ottawa City	Canada	pen	65
12	Babita Ghosh	Kolkata	India	pencil	75
13	Krish Pratt	London	England	eraser	30
14	Elizabeth Blunt	London	England	pencil	340
15	Nina Debrov	Amsterdam	Netherlands	Books	452

Examples to Implement in SQL Self Join

Here are a few examples to illustrate self joins in SQL.

Example #1

Find the pairs of customers who belong to the same city.

Code:

SELECT t1.Customer AS Customer1, t2.Customer AS Customer2, t1.City
FROM Customers t1, Customers t2
WHERE t1.ID <> t2.ID AND t1.City = t2.City
ORDER BY t1.City;

Output:

Explanation: In the above example, the self joins first joins the two instances of customers t1 and t2. It’s like a cartesian product of two tables. Then it filters the given rows based on the criteria that IDs of two customers should not be the same and they should belong to the same city. Finally, after filtering, the resulting records are sorted in ascending order by the city.

Example #2

Find customers who belong to the same city and have shopped for pencils

Code: 

SELECT t1.Customer AS Customer1, t2.Customer AS Customer2, t1.City
FROM Customers t1, Customers t2
WHERE t1.ID <> t2.ID AND t1.City = t2.City AND t1.Items_purchased = 'pencil'
AND t2.Items_purchased = 'pencil'
ORDER BY t1.City;

Output:

Example #3

Find the customers who belong to the same country and have shopped for less than 100

Code:

SELECT t1.Customer AS Customer1, t2.Customer AS Customer2, t1.Country, t1.Amount_paid , t2.
Amount_paid
FROM Customers t1, Customers t2
WHERE t1.ID <> t2.ID AND t1.City = t2.City AND t1.Amount_paid < 100 AND
t2.Amount_paid < 100
ORDER BY Country;

Output:

Example #4

Find the pair of customers who have spent more than 300 on Books

Code:

SELECT t1.customer, t1.Amount_paid,t2.customer, t2.Amount_paid
FROM customers as t1 , customers as t2
WHERE t1.Items_purchased = t2.Items_purchased
AND t1.Items_purchased = 'Books' AND t2.Items_purchased = 'Books'
AND t1.Amount_paid > 300 and t2.Amount_paid > 300 AND t1.ID <> t2.ID
Order by t1.Customer;

Output:

• When performing joins in SQL, we should always try to use table aliases which are abbreviations of the given tables. This helps in writing beautiful pieces of code.

Conclusion

SQL self-join is a statement that is used to join a table on itself. We should use it when we want to find relationships between records in the same table.

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