Relational modelling and SQL transaction thinking

≈ 45 min

Relational modelling and SQL transaction thinking

Relational modelling separates entities into tables with keys that identify rows and foreign keys that express valid relationships. An OrderLine should refer to an existing Order and Product; storing every product detail repeatedly in every order line invites inconsistent updates. Normalisation is a reasoning tool for reducing unwanted duplication while preserving the facts the system needs.

Constraints and transactions work together. A foreign key can reject an order line whose order does not exist; a check constraint can reject negative quantity; a transaction can group the valid rows so they commit together. Application code still needs clear errors and correct business rules, but the database provides an independent layer of integrity.

Worked reasoning. A shop database has Product(productId, stock), Customer(customerId), Order(orderId, customerId) and OrderLine(orderId, productId, quantity). Creating an order checks that the customer exists, inserts Order and OrderLine rows, reduces stock and commits as one transaction. If quantity would make stock negative, the rule prevents a partial order.

Exam lens. Name the entity, primary key, foreign key and constraint in a scenario. Explain which rule belongs in the database and which needs application-level policy.

Which statement is the most defensible principle for Relational modelling and SQL transaction thinking?

Model entities, keys and constraints so the database can enforce relationships before transaction code runs.

Treat an implementation detail as more important than the stated rule.

Assume one successful run proves the system is correct in every case.

Avoid recording assumptions, limits or failure conditions.

Enter the key term for Relational modelling and SQL transaction thinking. What relational constraint links a row to a valid row in another table?

A row in OrderLine refers to productId 88, but no Product 88 exists. What database rule should reject the insert?

A foreign-key constraint from OrderLine.productId to Product.productId should reject the invalid reference.

Use a larger class or broader permission without stating a rule.

Trust the first output and skip a boundary or failure check.

Hide the decision so affected people cannot inspect it.

Name the concise safeguard or principle that completes this lesson’s scenario: A row in OrderLine refers to productId 88, but no Product 88 exists. What database rule should reject the insert?