SQL TOPIC

SQL HAVING: filter groups after counting

HAVING filters on aggregate results, such as "countries with more than 3 customers".

WHERE filters rows before grouping; HAVING filters groups after. Use HAVING COUNT(*) > 1 to find duplicates.

You can use both in one query: WHERE status = 'paid' … GROUP BY … HAVING SUM(amount) > 1000.

Worked example

Which countries have more than three customers?

SELECT country, COUNT(*) AS customers
FROM customers
GROUP BY country
HAVING COUNT(*) > 3;

Result on the sample data

country	customers
Germany	4

Putting an aggregate condition in WHERE, which raises an error.
Using HAVING for row filters that belong in WHERE, which is slower and harder to read.