The Number of Users That Are Eligible for Discount

Start by restating the business rule exactly: which event makes a user eligible, whether dates are inclusive, and whether a user can appear only once. In SQL, join users to the relevant transactions or activity table on the stable user id, filter the qualifying status, amount, product, or date condition, then count distinct users. Prefer WHERE EXISTS (SELECT 1 ...) when the answer is only eligibility, because it naturally prevents duplicate rows from multiple qualifying events. If a grouped query is needed, group by user id and use HAVING for aggregate thresholds. This is typically O(U + E) work with indexes or a hash join, while storage depends on the engine. Test users with no activity, several qualifying purchases, borderline dates, cancelled rows, and null values. Explain which composite index supports the filter and join.