Qualify Function in DB2

In IBM DB2, the QUALIFY clause is used in conjunction with window functions to filter rows based on the result of those window functions. The QUALIFY clause is not directly supported in DB2. However, you can achieve similar functionality using a common table expression (CTE) or a subquery combined with window functions.

Here's an example that demonstrates how to achieve filtering similar to QUALIFY in DB2:

Suppose we have a table called sales with columns product_id, sales_date, and amount. We want to find the total sales amount for each product along with the rank of the product based on the total sales amount. We also want to filter out only those products whose total sales amount is greater than a certain threshold, say $1000.

WITH RankedSales AS (

    SELECT 

        product_id,

        SUM(amount) AS total_sales_amount,

        RANK() OVER (ORDER BY SUM(amount) DESC) AS sales_rank

    FROM 

        sales

    GROUP BY 

        product_id

)

SELECT 

    product_id,

    total_sales_amount,

    sales_rank

FROM 

    RankedSales

WHERE 

    total_sales_amount > 1000;

In this example:

- We're using a common table expression (CTE) named RankedSales to first calculate the total sales amount (SUM(amount)) for each product and assign a rank (RANK()) to each product based on the total sales amount.

- Then, in the outer query, we select from the CTE and apply a WHERE clause to filter out only those rows where the total_sales_amount is greater than $1000.

This achieves similar functionality to QUALIFY in other database systems by filtering the results based on a condition applied to the aggregated data. Although DB2 does not support the QUALIFY clause directly, the use of window functions along with CTEs or subqueries allows for similar result filtering.