How does MySQL handle filtering when both WHERE and HAVING clauses are present in a query?

Q: How does MySQL handle filtering when both WHERE and HAVING clauses are present in a query?

In MySQL, when both **WHERE** and **HAVING** clauses are used in a query, they filter data at different stages of execution. Understanding their roles ensures accurate filtering and correct results. **WHERE** – Filters individual rows from the table(s) before any grouping or aggregation occurs. It cannot use aggregate functions like SUM() or COUNT(). **HAVING** – Filters the results **after** grouping and aggregation. It can use aggregate functions to filter groups, such as `HAVING SUM(salary) > 200000`. When both are present, WHERE reduces the dataset first, then GROUP BY forms groups, and HAVING filters these groups based on aggregates. This separation ensures efficiency, as only relevant rows are grouped, and only relevant groups are returned. In practice, use WHERE to restrict rows before aggregation and HAVING to apply conditions on aggregated results. SELECT department_id, SUM(salary) AS total_salary FROM employees WHERE hire_date >= '2023-01-01' GROUP BY department_id HAVING SUM(salary) > 200000 ORDER BY total_salary DESC;