The Row Pairing Model

Concepts: 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