Which query computes totals per customer?

Question

Accepted Answer

Query A mixes DISTINCT with SUM(amount) without a GROUP BY - the database does not know how to group the rows, so it either errors or returns a single aggregate over all rows (depending on the engine). Query B is the correct approach: GROUP BY customer groups the rows, then SUM(amount) aggregates within each group. When you need aggregates per group, always use GROUP BY, not DISTINCT.