SQL Server: Recursive Query for Grouping Matched Products

In a relational database table named "matches," each record represents a match between two products. The goal is to create a "groups" table that captures these matches in a hierarchical structure. Specifically, the "group_ID" column should store the minimum product ID among those that belong to the same group.

To accomplish this, we can leverage the power of recursive queries in SQL. A recursive query performs multiple rounds of operations on the same data. Here's a solution that employs a common table expression (CTE) to track recursive operations:

WITH CTE
AS
(
    SELECT DISTINCT
        M1.Product_ID AS Group_ID,
        M1.Product_ID
    FROM matches M1
        LEFT JOIN matches M2
            ON M1.Product_Id = M2.matching_Product_Id
    WHERE M2.matching_Product_Id IS NULL
    UNION ALL
    SELECT
        C.Group_ID,
        M.matching_Product_Id
    FROM CTE C
        JOIN matches M
            ON C.Product_ID = M.Product_ID
)
SELECT * FROM CTE ORDER BY Group_ID

The base case of the recursion is when a product has no matches, in which case it forms its own group. This is captured in the first part of the CTE.

    SELECT DISTINCT
        M1.Product_ID AS Group_ID,
        M1.Product_ID
    FROM matches M1
        LEFT JOIN matches M2
            ON M1.Product_Id = M2.matching_Product_Id
    WHERE M2.matching_Product_Id IS NULL

The recursive part of the CTE finds all the matching products for each group. It iteratively adds these matching products to the group.

    SELECT
        C.Group_ID,
        M.matching_Product_Id
    FROM CTE C
        JOIN matches M
            ON C.Product_ID = M.Product_ID

Finally, we can retrieve the results in hierarchical order by sorting the CTE by the "Group_ID" column.

SELECT * FROM CTE ORDER BY Group_ID

This recursive query effectively produces the desired grouping of products in the "groups" table. The hierarchical relationship is captured by the "group_ID" column, ensuring that each row reflects a distinct product group.

By leveraging the recursive capabilities of SQL Server, we can efficiently extract complex hierarchical patterns and group related data in complex database structures.