What is the difference between WHERE and HAVING?

WHERE filters individual rows before grouping. HAVING filters groups after aggregation, so it can use aggregate functions such as COUNT or SUM, which WHERE cannot.

SELECT customer_id, COUNT(*) AS orders, SUM(total) AS spent
FROM orders
WHERE status = 'paid'            -- row filter, runs first
GROUP BY customer_id
HAVING SUM(total) > 1000;        -- group filter, runs after

The order is: FROM, WHERE, GROUP BY, aggregates, HAVING, SELECT, ORDER BY. That is why WHERE SUM(total) > 1000 is an error: when WHERE runs, the sums do not exist yet.

Put every condition that does not need an aggregate in WHERE, not in HAVING. It removes rows early so fewer rows are grouped, and it lets the database use an index.

-- better: filter before grouping
WHERE customer_id = 7
-- worse: same filter after grouping
HAVING customer_id = 7

Remember: WHERE filters rows, HAVING filters groups.