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This article explains what hierarchical queries in SQL are, why they are needed, and how to use them for working with tree-like data structures. Using a product catalog as an example, you’ll see how to create a table, set up relationships, fill it with test data, and write practical SQL queries to display and analyze the hierarchy.

This guide provides a comprehensive overview of SQL JOIN operations, with special attention to NULL value behavior in Oracle databases. Through practical examples using two test tables, we explore all major JOIN types and their specific behaviors when dealing with NULL values.