QUALIFY

What Is QUALIFY?

QUALIFY is a SQL clause that filters rows after window functions have been evaluated. It provides a direct way to select rows based on window results such as ROW_NUMBER(), RANK, or LAG.

Why QUALIFY Is Useful

QUALIFY makes it possible to filter window-function output without introducing extra CTEs or subqueries. This keeps queries shorter, easier to read, and often more efficient by avoiding additional processing layers.

Use in Data Warehousing

QUALIFY can be applied when working with Data Vault satellites that do not use end-dating, for example to:

Identify the valid records needed to derive an SCD1
Determine the relevant satellite versions for a PIT snapshot when ASOF JOIN is not available

Filtering directly on window-function results helps isolate the current or time-aligned entries required for these tasks.

Example select customer_key
      , order_date
      , total_price
   from orders
qualify row_number() over
       (partition by customer_key
        order by order_date desc) = 1;

Example select customer_key
      , order_date
      , total_price
   from orders
qualify row_number() over
       (partition by customer_key
        order by order_date desc) = 1;

Example select customer_key
      , order_date
      , total_price
   from orders
qualify row_number() over
       (partition by customer_key
        order by order_date desc) = 1;

Example select customer_key
      , order_date
      , total_price
   from orders
qualify row_number() over
       (partition by customer_key
        order by order_date desc) = 1;

Example select customer_key
      , order_date
      , total_price
   from orders
qualify row_number() over
       (partition by customer_key
        order by order_date desc) = 1;

Note for PostgreSQL

A proof-of-concept patch titled "Proposal: QUALIFY clause" was submitted to the PostgreSQL mailing list in July 2025.

Until official support is added, the equivalent logic must be implemented using a subquery or CTE: the window function is calculated first, and the filtering happens in the outer query.

Example select customer_key
      , order_date
      , total_price
   from orders
qualify row_number() over
       (partition by customer_key
        order by order_date desc) = 1;

Summary

QUALIFY streamlines the filtering of window-function results and avoids the need for additional query layers.
It leads to cleaner, more concise SQL and supports common data-warehousing patterns where identifying the current or time-appropriate record is essential.