Should a foreign key be created on the parent table or child table?

Question

What's the difference? If I have these two tables:

CREATE TABLE Account (Id int NOT NULL)

CREATE TABLE Customer (AccountId int NOT NULL)

And I want a foreign key linking the two, which of the following should I do and why?

Option 1:

ALTER TABLE [dbo].[Customer]  WITH CHECK 
  ADD  CONSTRAINT [FK_Accounts_Customers] FOREIGN KEY([AccountId])
  REFERENCES [dbo].[Account] ([Id])

Option 2:

ALTER TABLE [dbo].[Account]  WITH CHECK 
  ADD  CONSTRAINT [FK_Accounts_Customers] FOREIGN KEY([Id])
  REFERENCES [dbo].[Customer] ([Id])

Answer 1

Depends on context. Does every customer have a client? Which one is the parent? It seems like an Account has multiple Customers, in which case the reference belongs on the Customer table.

Now, that said, please call the entities CustomerID and AccountID everywhere. It may seem redundant on the primary table but the name should be consistent throughout the model.

Answer 2

I would use a foreign key from the child to the parent. The tell tale question is: what happens if you need to delete one of the entities?