SQL self-connection Self JOIN

May 16, 2021 SQL

SQL self-connection

A self-join is a regular join, but the table itself is connected.

Self JOIN syntax

SELECT column_name(s)
FROM table1 T1, table1 T2
WHERE condition;

Demonstrate the database

In this tutorial, we'll use the famous Northwind sample database.

Here are the selections in the Customers table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico

SQL Self JOIN example

The following SQL statements match customers from the same city:

Code example

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;