Obtaining data from multiple tables

Why Query Multiple Tables?

Relational databases split data across many tables to avoid duplication (normalization). Retrieving a complete picture — for example, an order along with its customer's name — usually means combining rows from two or more tables in a single query.

How It Works

SQL combines tables using a JOIN, which matches rows from one table to rows in another based on a related column, typically a foreign key referencing a primary key.

SELECT o.order_id, c.customer_name
FROM orders o
JOIN customers c ON o.customer_id = c.customer_id;

Why Testers Need This Skill

Application screens often display data assembled from several tables. Being able to write the equivalent multi-table query lets a tester independently verify that the UI is showing the correct, correctly-joined data.