There is a big difference between a primary key and a unique key in a database. Most people use the two terms interchangeably, but they are actually very different. A primary key is a column or set of columns that uniquely identifies each row in a table. A unique key is a column or set of columns that guarantees the uniqueness of each row in a table, but it does not have to be the primary key. Let’s take a look at an example to make it clearer.
What is Primary Key?
- Primary keys are a fundamental element of relational databases. A primary key is a column or set of columns that uniquely identify a row in a table. Primary keys can be composed of either a single column or multiple columns.
- When multiple columns are used, they are known as a composite primary key. Primary keys must contain unique values, and they cannot contain null values. In addition, each table can only have one primary key.
- Primary keys play an important role in ensuring the integrity of data in a database. They can be used to enforce relationships between tables and to prevent duplicate data from being entered into the database. Primary keys can also be used to create indexes, which can improve the performance of queries.
What is a Unique Key?
A Unique Key is a special type of key that can be used to unlock a unique door. Unlike a regular key, a Unique Key can only be used once. After it is used, the key disappears. Unique Keys are often given to people who need to enter a special room or area. For example, aUnique Key might be given to a person who needs to enter a room that contains a dangerous weapon.
Difference between Primary Key and Unique Key
There are two types of keys in a database- Primary key and Unique Key. Both the Primary key and Unique Key are used to uniquely identify a tuple in a relation.
- However, there are certain differences between the Primary Key and Unique Key which will be discussed in this article. A Primary key is a key that is used to uniquely identify a tuple in a relation. Primary keys can not have NULL values.
- There can only be one primary key in a relation. A unique key is a key that is used to uniquely identify a tuple in a relation. Unlike Primary Keys, Unique Keys can have NULL values. There can be more than one unique key in a relation.
- For example, assume that we have a table containing employee information such as employee id, employee name, date of birth, address, phone number, etc. The employee id attribute could be the primary key of this table as it is used to uniquely identify each employee.
The combination of phone number and email address could be the unique key of this table as it is also used to uniquely identify each employee. Thus, we see that there is only one Primary Key but there can be more than one Unique Key in a relation.
Conclusion
A primary key is always unique, but a unique key doesn’t have to be unique. A table can only have one primary key, but it can have multiple unique keys. As you design your tables and relationships in your database, remember the differences between these two important fields.
