The checkout count changes a wage total

Grandma's Marathon stores the official finisher count for each race. Ten marathons, and their counts add to 63,218. Join the results table to get the year labels, sum the same column, and the total becomes 2,442,010. Nothing failed. Each race's count was repeated once per result row, and SUM added every repeat.

SELECT s.branch_id, ROUND(SUM(s.hourly_wage), 2) AS wage_rate_sum
FROM staff s
GROUP BY s.branch_id
ORDER BY s.branch_id;

The same thing happens at the Tool Library. A staff member's hourly wage is one fact per person. Join staff to loans and that wage appears once per checkout; sum it and Priya's wage is counted thirteen times. The total grows every time someone borrows a hammer, which is not what a wage total should do.

The fix is not SUM(DISTINCT). Two different employees can earn the same wage and both must count. The fix is grain: aggregate each fact at the level where it is stored once, and join only after. Sum wages from staff alone. Count loans from loans alone. Combine the two summaries if the question needs both.

The habit that catches this is a single comparison: compute the total from the parent table by itself, then from the join. If they differ, you have multiplied something.