A foreign key is a column or a set of columns in one table that refers to the PRIMARY KEY (or UNIQUE key) of another table. It establishes a relationship between the two tables and ensures that the data remains consistent.

1. Why Foreign Keys Are Important

1
They enforce referential integrity, ensuring relationships between tables remain valid.
2
They prevent invalid data — e.g., you cannot insert an order for a non-existing customer.
3
They maintain consistency across related tables.
4
They enable cascading actions (ON DELETE and ON UPDATE).

2. What Foreign Keys Enforce

1
Parent row must exist before child rows can reference it.
2
Child rows cannot refer to non-existing parent rows.
3
Optional cascading actions such as deleting or updating related rows.

Example: Using a Foreign Key

In this example, each order must be linked to a valid customer. MySQL prevents inserting an order with a customer_id that does not exist in the customers table.

Scenario Questions

0-2 years experience

2-5 years experience

5-8 years experience

8+ years experience

Follow-up Questions

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