Join a table with itself.
✍️ 3 exercises + code challenge in this lessonA self join joins a table to itself, using two different aliases. It is useful to compare rows within the same table.
The example finds pairs of customers who live in the same city. The condition a.CustomerID < b.CustomerID avoids pairing a customer with themselves and listing each pair twice.
Mastering SQL Self Join helps you write clean and reliable SQL code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.
SELECT a.CustomerName AS Customer1,
b.CustomerName AS Customer2,
a.City
FROM Customers a
JOIN Customers b
ON a.City = b.City AND a.CustomerID < b.CustomerID;Check what you learned in “SQL Self Join”.
Join the table to itself with two aliases.
FROM Customers a JOIN Customers ON a.City = b.City
Why use a self join?
List pairs of customer names (Customer1, Customer2) in the same City, each pair once (a.CustomerID < b.CustomerID), sorted by Customer1.
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