SQL query to match similar customers

I’m trying to find the query in order to match similar customers.

To simplify the situation consider this scenario:

I have a table which contains a customer name and product purchased. customer name can have multiple purchases of same and different products. So firstly I can take distinct customer name and product name, so I see all customers and all products they purchased at least once.

Now I want a query to show me a sort of matching customers, according to the product they both purchased, so I want to count the similar products they purchased.

So I want to see for each pair of customers (pairing all the table) the amount of similar product they purchased.

Lets say the raw data is:

CustomerName | ProductName
A            | 1
A            | 2
A            | 1
A            | 3
B            | 1
B            | 2
B            | 4
C            | 2

Then I want to see the result of:

CustomerName1 | CustomerName2 | CountSimilarity
A             | B             | 2
A             | C             | 1
B             | C             | 1

And so on for all pairs of customers that have at least 1 similar product purchasing

Any suggestions how to approach this query?

The environment is SQL Server.

Thanks

Answer

Here is a self join approach:

SELECT t1.CustomerName, t2.CustomerName, COUNT(*) AS CountSimilarity
FROM yourTable t1
INNER JOIN yourTable t2
    ON t1.ProductName = t2.ProductName
WHERE
    t1.CustomerName < t2.CustomerName
GROUP BY
    t1.CustomerName, t2.CustomerName;

Two records are joined together above if their products match. Note that the inequality in the WHERE clause ensures that customer pairs do not appear in duplicate.