How Recursive CTEs Let SQL Navigate Hierarchical Data of Unknown Depth

Hierarchical data — such as employee reporting chains, nested folders, or threaded comments — poses a challenge for standard SQL queries because the depth of the hierarchy is unknown in advance. A Common Table Expression (CTE) is a named, temporary query block defined using the WITH keyword, making complex queries more readable. A recursive CTE extends this concept by allowing the named block to reference itself, enabling SQL to repeatedly traverse one level of a hierarchy until no further connections exist. It is structured in two parts joined by UNION ALL: an anchor query that runs once to establish the starting point, and a recursive part that keeps finding the next layer of connected records. This approach eliminates the need for hardcoded joins and ensures no records are missed regardless of how deep the hierarchy grows.