The Row Pairing Model

Concepts covered: sqlSelfJoin

Before learning specific join syntax, you need to understand the fundamental concept: how SQL matches rows from different tables. A join creates pairs of rows from two tables. Each row from the first table is tested against each row from the second table. When rows match a condition, they form a pair in the output. This is the core mental model for understanding joins. SQL does not copy data or modify the original tables. It produces a new result set made of matched row pairs. Visual: How Pairs Form Consider two small tables: When we join on customers.id = orders.cust_id, SQL pairs each customer with their matching orders: Notice that Alice appears twice because she has two orders. The join created one row for each matching pair: Alice-Order101 and Alice-Order102. The join concept itself h

About This Interactive Section

This section is part of the Joins: Beginner lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live PostgreSQL database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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