Oracle Database 26ai: Filtering Analytic Function Results with the QUALIFY Clause

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Suppose we want to display, from the employees table, only the employee with the highest salary in each department.
Before version 26ai(23.26), we usually had to use a subquery to achieve this result:

SELECT *
FROM (
  SELECT employee_id,
         first_name,
         department_id,
         salary,
         RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS Rank_Per_DEP
  FROM employees
)
WHERE Rank_Per_DEP = 1;

In version 26ai, you can achieve the same result without using a subquery, thanks to the QUALIFY clause:

SELECT employee_id,
       first_name,
       department_id,
       salary,
       RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS Rank_Per_DEP
FROM employees
QUALIFY Rank_Per_DEP = 1;

Therefore, the QUALIFY clause is used to filter the results of analytic functions.
It eliminates the need for a subquery and improves both the readability and efficiency of the query.

If you try to use this clause in earlier Oracle versions, you’ll encounter the following error:

ERROR at line 7:
ORA-03049: SQL keyword 'RANK_PER_DEP' is not syntactically valid following
'...FROM employees
QUALIFY '
Help: https://docs.oracle.com/error-help/db/ora-03049/

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