How to optimize SQL query with multiple selects

Question

I have a query that uses multiple SELECTS, and I need to optimize this but I have no clue how to do this.

Query:

SELECT e.last_name, e.salary, t1.PROMEDIO 
FROM employees e, 
     (
       SELECT e.department_id, AVG(e.salary) PROMEDIO 
       FROM employees e 
       GROUP  BY e.department_id
     ) t1 
WHERE e.department_id = t1.department_id 
     AND e.salary < t1.PROMEDIO;

Answer 1

If I'm understanding your question correctly, you are trying to return all employees whose salary is less than the average salary per department. If so, you can use the window function avg() over() :

select * 
from (
    select last_name, department_id, salary, 
         avg(salary) over (partition by department_id) avgsalary
    from employees
) t
where salary < avgsalary

• Online Demo

Answer 2

It is better to write this query using window functions:

select e.* 
from (select last_name, department_id, salary, 
             avg(salary) over (partition by department_id) as avgsalary
      from employees e
     ) e
where salary < avgsalary;

For performance, you want an index on employees(department_id, salary) .