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It’s all fine by me, whatever floats your boat as they say.

There are lots of discussions and ideas about the best way to determine what the best primary key of these tables should be, and pros and cons of all of the various approaches, but overall, that is not really what I am addressing.

That handles that problem, our data now has integrity, so we seem to be all set, right?

Well, let’s remember the definition of what a primary key really is.

It's possible to constrain data properly, it's efficient, it's easy to do, and it simply be done or you should not really be working with a database in the first place -- you are forgoing a very important advantage it provides.

So, to handle that issue with this table design, we need create a unique constraint on our Customer ID/Product ID columns: Now, we are guaranteed that there will only be exactly one row per combination of Customer ID and Product ID.

We also do not need an additional identity column that really serves no purpose.

We have not only simplified our data model physically, but we’ve also made it more logically sound and the primary key of this table accurately explains what it is this table is modeling – the relationship of a Customer ID to a Product ID.As I often try to do, I will attempt to approach this topic from a slightly different perspective.Let's start things off with what I feel is a good interview question: aware of this.I once worked with a consultant who kept claiming that importing data into his system was complicated, because his database used “primary keys”.It was very confusing (yet humorous) trying to discuss things with him because he kept confusing primary keys with identity columns.Let’s also add a simple “Order Limit” column which indicates how many of that product they are allowed to order. I can only surmise that it is because of the lack of understanding what a primary key of a table really is, and that it can be something other than an identity and that it can be comprised of more than just a single column. This table allows me to enter the following data: In the above data, what is the order limit for customer ID #1, product ID #100? Remember, our primary key is just an identity, which does not constrain anything. ) that the data will be always be OK and there will be no duplicates. But even if you think that only one single form on one single application get in and out of your system in different ways.

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