One-to-many and one-to-one

🏠The everyday picture

A mother can have several children; each child has one mother (one-to-many). Each person has one passport, and each passport belongs to one person (one-to-one).

📘What it really is

One-to-many is the most common relationship: one customer has many orders; each order has one customer.
The foreign key goes on the "many" side: orders.customer_id.
Never the other way round (a customers.order_ids list), which can't be enforced or queried properly.
One-to-one: one row relates to exactly one row in another table, for example users and user_profiles, or employees and bank_details kept separately for security.
The foreign key goes on either side, with a UNIQUE constraint so it can't become one-to-many: user_id integer NOT NULL UNIQUE REFERENCES users(id).
Often one-to-one tables can simply be merged; separate them for a reason (security, optional data, very large columns).
Optional relationships: a nullable foreign key means "may have one": an enquiry's assigned_to is NULL until someone is assigned.

🖱️Try it

Add an addresses table where a customer can have many addresses (home, office), and a customer_kyc table with one-to-one sensitive details. Decide which side gets the foreign key in each case, and add the UNIQUE constraint where needed.

✅Quick check

One customer has many orders. Which table gets the foreign key?

Show the answer

orders, with a customer_id column.

How do you stop a one-to-one relationship becoming one-to-many?

Show the answer

Put a UNIQUE constraint on the foreign key column.