SQL Self Join (Self-join)

SQL Self Join Description

A self-join is a regular join where a table is joined to itself.

Self Join Syntax

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

T1 and T2 are different aliases for the same table.

Demonstration database

In this tutorial we will use the famous example database "Northwind".

Below is a sample from the table "Customers" ("Customers").

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.	5021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	5023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SQL Self Join Example

The following SQL statement selects customers from one city (City):

Run SQLSELECT 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

MySQL dialect uses "!=" instead of "<>".