Aggregation and GROUP BY

Aggregate functions reduce a set of rows to a single value: COUNT, SUM, AVG, MIN, MAX. Used alone they summarise the whole table.

GROUP BY splits the table into buckets first, then applies the aggregate to each bucket. Every column in the SELECT list must either be grouped or aggregated.

WHERE filters rows before grouping; HAVING filters the groups afterwards. If your condition mentions an aggregate, it belongs in HAVING.