Detecting unexpected fan-out

Question

Accepted Answer

A doubled row count is the hallmark of a many-to-one join where you expected one-to-one. Each row in the driving table matched two rows in the joined table, producing two output rows for every one input row. The fix depends on intent: if you only need one row per match, deduplicate the joined table first (subquery with DISTINCT or ROW_NUMBER), or aggregate before joining. Always verify the cardinality of both sides before writing a JOIN - COUNT(DISTINCT key) vs COUNT(*) in the joined table quickly reveals duplicates.