Why this lesson matters

Understand foreign keys and reason about it with a concrete relational example. Primary keys give rows stable identity, foreign keys protect references between tables, and relationship cardinality determines where those keys belong.

How to reason about it

  • For Foreign Keys, the outcome to verify is: Run the statement against sample data and inspect the result before moving to the next case.
  • In Foreign Keys, keep this failure controlled: Storing related names or labels instead of keys creates duplicates and makes updates unable to preserve referential integrity.
  • Foreign Keys practice target: Model customer, order and product relationships with keys and identify one-to-many and many-to-many cardinality.
Diagram

Foreign-key relation

Practical walkthrough

In the Foreign Keys walkthrough: Run the statement against sample data and inspect the result before moving to the next case.

example.sqlsql
CREATE TABLE orders (
  id BIGINT PRIMARY KEY,
  customer_id BIGINT NOT NULL REFERENCES customers(id)
);

Practice it yourself

Practice

Foreign Keys exercise

Model customer, order and product relationships with keys and identify one-to-many and many-to-many cardinality.

  • Record the expected result before execution
  • Test one valid path and one lesson-specific failure path
  • Explain in two lines which boundary owns the decision

Summary