Advantages / disadvantages of simple vs compound primary keys

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Question :

I am trying to model for a database. For one part of the problem I have an entity Area, which contains Departments, and each department is assigned specific functions. I have the following.

I have thought of two ways to do relationships. The first is by creating a self-incrementing key for the Department table, assuming that department names can be repeated in different areas. I do the same for the Function table, having an incrementable key as identifier, and making a unique key for the combination (Department_ID, Name).

The second way is to place the combination of fields (Department_ID, Name) as the primary compound key of Department, and the same case for the Function table, placing the fields (Department_ID, Name) as the compound primary key.

What are the advantages or disadvantages of using a simple key over a compound key as in the first case and how does it affect the use of unique keys? Is it a correct way to do it?


Answer :

Short answer

From a logical perspective, there are no inherent disadvantages to a composite key. It exists. If it is required to maintain uniqueness/data integrity, you must use it.

From a physical implementation perspective, a composite key may create fragmentation of the b-tree and clustered index (if used/supported) as inserts are not guaranteed to append to the end of the table. Most commercial database engines are pretty good about handling/managing fragmentation (and some always append new rows to new pages) so this shouldn’t be your reason for choosing one over another except in a very narrow set of circumstances.

Longer Answer

So let’s back up a moment because I think there’s a crucial thing about primary keys that’s missed by most practitioners:

A primary key must enforce uniqueness of the data.

An id column does not enforce uniqueness as it is system-generated, not from the data.

So it doesn’t make sense to tack an id on every entity at the beginning of data modeling because it will prevent you from understanding the actual structure of the data. We can apply id columns later, once the actual keys are known. The id will be the surrogate for the prior primary key, and that key will become an alternate key.

So I can see from your diagram that you are at least aware of that later fact for Function, but have not applied that to Area or Department.

So let’s begin by removing the row identifiers from the equation. This is what you would have:

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At this point it would be natural to ask:

  • Is a function independent of all departments?
  • Can more than one department fulfill the same function?

In the first case, our model would then become:

enter image description here

But if the second held true, then the model instead would be:

enter image description here

So you can see, there is a lot of work to be done before we start thinking about whether or not to replace the existing primary key with a surrogate.

We may decide that Area_Name is too wide to be useful in practice. We can make that an alternate key and substitute either a human-readable code/short name (preferred) or system-generated integer (if you really must). We could decide similarly for Department_Name.

So let’s implement the second model, replacing the wide keys with surrogates:

enter image description here

In this case, DepartmentFunction has the key (Department_Id,Function_Id) which is:

  • Compact
  • Preserves the relation to Department, which may be needed for integrity later in the data model

So there is no compelling reason to replace it.

One thing I would stress is that only use integer surrogates as a last resort. If there is an shorthand/code that can be used this is preferable as:

  1. It is human readable
  2. May already be in common use
  3. May reduce the number of joins required
  4. May be more compact if the length is less than 4
  5. Reduce data errors (integer columns get swapped more than some would like to admit)

The second way is to place the combination of fields (Department_ID, Name) as the primary compound key of Department

This is wrong because:

  1. provides neither Department_ID nor Name uniqueness
  2. increases joining expression complexity and size (in both tables!) and hence decreases joining performance without any visible profit

What are the advantages or disadvantages of using a simple key over a compound key

We speak now only about primary key, is it?

Compound primary key always must be natural. Even when it is a combination of 2 columns which are references to synthetic PKs of another tables (M:N relation junction table). Compound primary key which contains synthetic component makes no sense – the row uniqueness over a table may be controlled by this synthetic expression only. When the uniqueness of some natural expression needed then regular unique index is enough.

You could and generally it’s a good idea to have both. A simple auto-increment key guarantees uniqueness and generally is most performant for joining on, but a compound key helps expose the logical purpose of your entities and their relationships (as long as it’s natural and not forced).

As far as choosing one as the actual PrimaryKey constraint, if your tables end up having both a natural compound key and a unique auto-increment field, you can set the compound key as the PrimaryKey constraint but separately set the auto-increment field as a unique clustered index. You can also create a nonclustered index on the PrimaryKey. (Technically you can also let the compound key be set as the clustered index and do a unique nonclustered index on the auto-increment field too.) This way you have performant joins on either key.

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