Which of these two methods is standard when creating a 1 to many database relationship

database-design

If I have a customer that can have many addresses, I can create an Address table with columns Street, Town etc. and CustomerId. Then I can insert multiple records to have multiple addresses per customer.

Alternatively I can create multiple addresses and give them all the same AddressId, then in my customer table I can have an AddressId (so you'd do SELECT * FROM Address WHERE Address.AddressId = Customer.AddressId).

Which of these is better, is there some reason why you'd use one over the other, or is one of them just silly?

Best Answer

Alternatively I can create multiple addresses and give them all the same AddressId.

This is not a good idea at all. The AddressId supposes to be a primary key in your Address table.

Foreign Key

Use foreign Key relationship between customer and address tables, if customer may have multiple addresses associated to him/her.

Do enforce foreign key relationship between customer and its addresses by CustomerId being a primary key in Customer table and foreign key in Address table. This will ensure that no orphan records will be left, when customer record is deleted or in-activated.

One-to-many relationship diagram:

enter image description here

In the case if same address can be assigned to different customers, aka many-to-many between customer and addresses, do use a third table CustomerAddress (junction table). Junction table should save pair of (CusomerId, AddressId) and any other related information specific for this combination.

For example, in below diagram we have many to many relationship between Orders and products.

For more references on DB design patterns look here.

Many-to-many relationship diagram:

enter image description here

Related Question