SQL: count and group with COUNT and GROUP BY

"How much did I sell this month? And above all: what really brings money in? Because rice, I sell a lot of it, but the bag is expensive too." Until now you were narrowing and sorting rows. Here they disappear: three hundred and twenty sales fold into a handful of figures. It is SQL's second wall after the join, and it gets a workshop of its own rather than a sub-chapter. COUNT, SUM, DISTINCT, GROUP BY, HAVING, and SQL's most frequent error message, the one that complains about a column missing from the GROUP BY. You will trigger it on purpose, and you will understand why PostgreSQL refuses rather than inventing an answer where other databases used to pick a row at random without saying anything. You will also learn to filter the groups themselves, which WHERE cannot do, and you will see why: when WHERE does its work, the groups do not exist yet. The workshop ends with a challenge with no solution given: find the product that brought in the most money in August. The answer is in the guide, folded under a ribbon you only pull after searching. And a surprise awaits you in the figures: a category comes second when it should not. You already know the cause. Wrong data does not show in the data, it shows in the result that surprises you, and a surprising result is checked before it is announced. That is the professional reflex this workshop builds.