HAVING vs. WHERE in SQL

It's common for developers to get confused about the difference between the HAVING and WHERE clauses – they're pretty similar after all.

The difference, though, is straightforward enough:

• A WHERE condition is applied to all the data in a query before it's grouped by a GROUP BY clause.
• A HAVING condition is applied only to the grouped rows that are returned after a GROUP BY is applied.

This means that if you want to filter based on the result of an aggregation, you need to use HAVING. If you want to filter on a value that's present in the raw data, you should use a simple WHERE clause.

SELECT class_id, COUNT(id) AS class_size
FROM students
WHERE ...
GROUP BY class_id
HAVING ...