Should I add transitive foreign keys?

Question

Simple example: there is a table of customers.

create table Customers (
  id integer,
  constraint CustomersPK primary key (id)
)

All the other data in the database should link to a Customer, so e.g. Orders looks like this:

create table Orders (
  id integer,
  customer integer,
  constraint OrdersPK primary key (customer, id),
  constraint OrdersFKCustomers foreign key (customer) references Customers (id)
)

Suppose now there is a table linking to Orders:

create table Items (
  id integer,
  customer integer,
  order integer,
  constraint ItemsPK primary key (customer, id),
  constraint ItemsFKOrders foreign key (customer, order) references Orders (customer, id)
)

Should I add a separate foreign key from Items to Customers?

...
constraint ItemsFKCustomers foreign key (customer) references Customers (id)

A picture instead: should I add the dashed line/FK?

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Edit: I have added primary key definitions to the tables. I'd like to re-iterate on the point I made above: the database is basically siloed by customers, as a correctness / security measure. Therefore, all primary keys contain the customer ID.

Answer 1

I think this is the original idea.

First thing to notice is that the PK on the LineItem table has three attributes {CustomerID, CustomerOrderNo, OdrerItemNo}, as opposed to just two in your example.

Second thing to note is the confusion resulting from the use of the generic id name for an attribute.

The CustomerOrderNo should ideally be (1,2,3..) for each customer and OrderItemNo (1,2,3 ...) for each order.

Well, this is nice if possible, but requires a query looking for the previous max value, like

select max(CustomerOrderNo)
from Order 
where CustomerID = specific_customer ; 

which is often not preferred in high-transaction-volume environments, so it is common to see these replaced by an auto-increment, essentially serving the same purpose. It is true that this auto-incremet is now unique, hence it can be used as a KEY -- but you may choose to look at it as a necessary compromise for the OrderItemNo.

So, with some renaming CustomerOrderNo -> OrderNo and OrderItemNo -> ItemNo you may arrive to this model

So now if you look at the Order the following are unique

{OrderNo}             -- PK
{CustomerID, OrderNo} -- superkey,  AK on the diagram.

Note that {CustomerID, OrderNo} is propagated to the LineItem to serve as a FK.

If you squint a bit, this is close to your example, but with PKs {ItemNo} and {OrderNo} only -- as opposed to two column PKs from your example.

Now the question is, why not simplify to something like this?

Which is fine, but introduces PATH DEPENDENCE -- you can not join LineItem with Customer directly, must use Order in the join.

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I prefer the first case when possible -- you choose your favourite. And obviously, there is no need for direct FK from LineItem to Customer in these three cases.

Answer 2

The "item" shouldn't reference the "customer" directly, because this is implied by the item's "order". So, you won't need the "customer" columns on the "items" table at all.

The item's relation to the customer is ensured with the existing foreign key.

If orders.id is an identity column, consider removing items.customer alltogether.

Answer 3

From a data integrity point of view the transitive foreign key is redundant. And in principle, with a well written database management system, it wouldn't make any difference to performance, since the DBMS could see that the transitive FK is implied by the other two, so would not need to check it separately.

Sadly this is not the case in the real world. I found that with Microsoft SQL Server, the query planner wasn't smart enough to spot that if there is a foreign key from table A to table B, and from B to C, then it can assume a foreign key from A to C when working out cardinality. Adding the transitive FK explicitly, even though redundant, improved performance on some queries.

What I settled on is to add the 'underlying' foreign keys for data integrity, and to decide on a case by case basis whether to add the 'transitive' FKs, depending on query performance.