Answer
A foreign key requires values to match a referenced unique or primary key. • It preserves referential integrity between related tables. • The referencing value may be NULL unless another constraint forbids it. • Update and delete actions define what happens when the referenced row changes.
💡 SQL Example
CREATE TABLE orders (order_id integer PRIMARY KEY, customer_id integer REFERENCES customers(customer_id));
Result
CREATE TABLE
⚡ Quick Revision
A foreign key keeps references aligned with a candidate key in another table.