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What is the difference between a composite key, primary key and foreign key?

By Justin Stoltzfus | Last updated: May 5, 2022

The difference between a composite key, a foreign key and a primary key is a good illustration of the complex and byzantine nature of relational database standards that make database administration an advanced job role requiring specialized skills.

Primary Key

First, a primary key uniquely identifies each record in a database table. Any individual key that does this can be called a candidate key, but only one can be chosen by database engineers as a primary key.

Composite Key

Next, there's the composite key, which is composed of two or more attributes that collectively uniquely identify each record.

An example would be a list of homes on a real estate market. In a well-ordered database, there should be a primary key that uniquely identifies each record.

How this works may have to do with the sophistication of the database.

In some cases, the homes may only be uniquely identified by a mortgage number — all other data (towns, streets, house numbers) is not unique to each record. The mortgage number would be the primary key. Suppose, however, that an MLS realtor’s listing technology assigns its own unique numbers to the records in the table.

Then, there will be two keys that developers might identify as “candidate keys”:

  • The mortgage number.
  • The MLS number.

One of them will qualify as the “primary key” in what some would consider an arbitrary way.

A composite key, then, would be the combination of two keys.

For example: the combination of house number and street might qualify as a composite key, given that the market listings are local. If so, then when someone searches using both the house number and the street, they should only get one single record returned.

Foreign Key

Meanwhile, if there is a key in a linked table, such as a buyer’s table that references the primary key, that will be a foreign key.

While a primary key and a composite key might do the same things, the primary key will consist of one column, where the composite key will consist of two or more columns.

The relationship between a primary key and a foreign key is quite different. The key thing to understand here is that the primary key in one database table becomes a foreign key in another database table, and vice versa.

A foreign key in a database table is taken from some other table and applied in order to link database records back to that foreign table.

The foreign key in the database table where it resides is actually the primary key of the other table.

Here's an example to make this clearer: If we have a database table labeled, say, "orders," and we have a foreign key that's labeled "customers," we could perhaps link that foreign key to a primary key, identifying each customer in a separate table.

In sum, all of this interconnection of key categories shows the detailed nature of creating sophisticated patterns and relationships in relational database systems.

The database engineer knows these categories and attributes intimately, and applies them in order to help ensure that the SQL queries and other data retrieval methods triggered by end users are effective.

Along with database normalization and promoting structured data, these key relationships are core parts of database system integrity and data storage design best practices.

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Database Data Management

Written by Justin Stoltzfus | Contributor, Reviewer

Profile Picture of Justin Stoltzfus

Justin Stoltzfus is a freelance writer for various Web and print publications. His work has appeared in online magazines including Preservation Online, a project of the National Historic Trust, and many other venues.

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