Recursive CTEs Explained: The SQL Feature That Handles Unlimited Hierarchy Levels

Recursive Common Table Expressions (CTEs) solve a longstanding SQL limitation: traversing hierarchical data of unknown depth, such as org charts, category trees, or folder structures. A recursive CTE consists of two parts joined by UNION ALL — an anchor member that defines the starting row and a recursive member that repeatedly joins back to the CTE's own output until no new rows are returned. The technique works in PostgreSQL, SQL Server, and MySQL 8.0 or later, but is unsupported in older MySQL versions. Developers should use UNION ALL rather than UNION to avoid costly deduplication, and must guard against cyclic data — where relationships loop back on themselves — by tracking visited IDs or capping recursion depth. While powerful, recursive CTEs recompute the full traversal on every query, so alternative design patterns may be preferable for very large or frequently queried hierarchies.