SQL Self Join
SQL Self Join
A self join is just a normal join except that it joins the table to itself.
Self Join Syntax
SELECT column_name(s)
FROM table1 T1, table1 T2
WHERE condition;
T1 and T2 are different table aliases for the same table.
Demo Database
We’ll be using the well-known Northwind sample database in this tutorial.
A sample from the “Customers” table is shown below:
| 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
Customers from the same city are matched using the SQL statement that follows:
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;
