SQL JOINs Explained: How Row Counts Change and Why Silent Errors Happen

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A detailed technical guide explains how SQL JOIN operations determine output row counts, addressing two common silent errors: dropped rows and duplicated rows. The core rule is that a JOIN produces one output row for every matching pair between two tables, meaning results can be smaller or larger than either source table. The guide uses a real-world dataset of 175 games and over 41 million Steam player reviews to demonstrate these mechanics across all six JOIN types. Readers are advised to count rows before and after every JOIN to catch mistakes that might otherwise surface undetected in reports. Understanding a table's 'grain', or what a single row represents, is presented as essential before writing any JOIN query.

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