Hierarchical Queries and Tree Traversal

Many real-world datasets contain parent-child relationships: genre taxonomies, organizational charts, category trees, threaded comments. SQL tables store these as self-referencing foreign keys, where each row points to its parent. The challenge is that the depth of the tree is unknown at query time, so you cannot solve these problems with a fixed number of JOINs.

Recursive CTEs are the standard tool for traversing these structures, and they show up regularly in interviews at companies with enterprise, HR, or e-commerce systems.

This chapter assumes you are comfortable with recursive CTE syntax (anchor member, recursive member, UNION ALL). If not, review the Recursive CTEs chapter first.