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A correlated scalar subquery can turn a single-table read into repeated scans. This guide shows how to rewrite it as a window function in MySQL 8.0+, validate the plan, and recover safely.
MariaDB 10.2+ window functions replace brittle self-joins for running totals and top-N-per-group queries. A worked example shows ROW_NUMBER() and SUM() OVER with explicit framing, plus EXPLAIN checks to catch temporary-table spills before production.