SQL: What is the difference between GROUP BY and HAVING?

GROUP BY vs HAVING

These two clauses serve different purposes in a query:

• GROUP BY collapses rows into groups based on the values of one or more columns, so you can apply aggregate functions (COUNT, SUM, AVG, etc.) per group instead of over the whole table.
• HAVING filters the groups produced by GROUP BY, based on conditions involving aggregate functions. It runs after grouping/aggregation has happened.

This is different from WHERE, which filters individual rows before any grouping or aggregation occurs — and that's why you can't use aggregate functions in a WHERE clause.

Example:

SELECT dept, COUNT(*) 
FROM employees 
GROUP BY dept 
HAVING COUNT(*) > 5;

Here:

1. GROUP BY dept groups all employees by department.
2. COUNT(*) counts employees in each group.
3. HAVING COUNT(*) > 5 keeps only the departments where that count exceeds 5.

If you try:

SELECT dept, COUNT(*) 
FROM employees 
WHERE COUNT(*) > 5 
GROUP BY dept;

Postgres (and most SQL engines) will raise an error, since COUNT(*) isn't available yet at the WHERE stage — the rows haven't been grouped/aggregated.

Summary:

• WHERE → filters rows before grouping.
• GROUP BY → groups rows for aggregation.
• HAVING → filters groups after aggregation, typically using aggregate conditions.