intro
Primary key vs. Foreign key: What are they, how do they work, and which one should you choose for your database operations? Learn here.
Ahh... Keys. First and foremost, keys lock and unlock doors—doors that safeguard the entrance to many precious buildings. Keys are used so often that many of us forget that they’ve locked doors in the first place!
In the database world, however, things are a little different. Here, “keys” mostly refer to indexes; indexes that enhance SELECT query performance. In the database world, primary key vs. foreign key is a frequent point of discussion for many developers and DBAs alike: what are these keys? How do they differ and what kind of key should I use to enhance my database performance? Read on.
What Are Keys in a Database?
The first question we need an answer to is what we mean by saying “keys” in the first place. Everything’s simple. Keys denote just another term for indexes in our database. Various database management systems offer various keys available for us to use.
In this blog, we’re using MySQL Server and its counterparts MariaDB Server and Percona Server as examples, but other database management systems also offer the following variants:
Enough about index types, though; you probably know them all by heart. Let’s get into the main question—what is a primary key? What is a foreign key? What makes a primary key vs. foreign key comparison possible?
SQL Primary Keys
Let’s dig into the primary key vs. foreign key comparison. You’ve surely heard these two terms before, right?
In many cases, most of you would’ve heard of the term “primary key” when defining a column to increment automatically and that’s kind of the concept of primary keys in the first place. In any database management system, a primary key is a unique identifier for rows in a table.
Usually, columns with a primary key can only consist of integer values and that’s for a good reason—primary keys increment column values by 1 each time a new row is added and that means that they solve a couple of issues in one go:
In other words, primary keys are SQL indexes that uniquely identify rows—primary keys will usually consist of unique, automatically incrementing integers and will never ever have a NULL value alongside them.
SQL Foreign Keys
In simple terms, foreign keys refer to a singular column or several columns that refer to a PRIMARY KEY in another table. Keys referring to other keys are a big part of the part of the title itself — “Foreign” means “not a frequent guest.” And, indeed, foreign keys aren’t frequent guests in our database either.
Foreign keys are mostly used to enhance relationships between tables—such indexes are indexes that are linked to a column in another table. As such, a rule is being established—a value can only be added or updated in a column X if the same value exists in a column Y in another table. What a strange guest, right?
Primary Key vs. Foreign Key in SQL
Now, towards the real discussion—What’s the main primary key. vs a foreign key differences? Is there any particular difference to begin the comparison with?
Indeed, attentive readers will realize that there are a couple:
In short, the core difference between a primary key vs. foreign key is that a primary key is used to uniquely identify rows and is almost exclusively used on integer columns, while a foreign key is used to “connect” rows in two tables. When a foreign key is in use, rows existing in a table X can only be added or updated if the relevant column in a table Y holds the same value.
Keys and SQL Clients
Now that you know what the primary key vs. foreign key difference is, it’s time to explore the opportunities posed by these SQL index types.
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Summary
The primary key vs. foreign key comparison is simple—both primary keys and foreign keys are indexes with a distinct difference: a primary key is used to uniquely identify rows and is almost exclusively used on integer columns with automatically incrementing values (see example above), while a foreign key is used to “connect” rows in two tables.
Frequently Asked Questions
What is a primary key in SQL?
A primary key is a type of index that is used to uniquely identify rows in a column and is almost exclusively used on integer columns.
What is a foreign key?
A foreign key is a type of index that is linked to a column in another table. That allows such indexes to “connect” with rows in another table.
Should I use primary keys or foreign Keys?
Primary keys should be used whenever you need values in a column to increment automatically—foreign keys should be used whenever you need values existing in table X to only be updated if the value in table X’s foreign key column exists in the indexed column in table Y.
Where can I learn more about databases?
Check the Database Dive YouTube channel out—they make videos on database performance, availability, and security often. Free of charge!
Where can I grab a free trial for DbVisualizer?
Click here. The free trial is valid for 21 days. The link is no secret — explore the possibilities offered by a primary key vs. foreign key comparison, then make sure to share it with your colleagues!
