Difference between Primary Key and Unique key
Keys in MySQL are the column or set of columns used to construct a relationship between one or more than two tables. They are also used for accessing records from the table. Both keys provide a guaranteed uniqueness for a column or a set of columns in a table or relation. The main difference among them is that the primary key identifies each record in the table, and the unique key prevents duplicate entries in a column except for a NULL value. In this article, we are going to compare essential differences between Primary and Unique Keys based on various parameters. Before making a comparison, we will discuss in brief these keys.
What is Primary Key?
The primary key is a unique or non-null key that uniquely identifies every record in that table or relation. The primary key column cannot store duplicate values that mean primary key column values are always unique. It is also called a minimal super key; therefore, we cannot specify more than one primary key in any relationship. A primary key column of one table can be referenced by a foreign key column of another table.
For example, we have a table named students with attributes such as Stud_ID, Roll_No, Name, Mobile, and Email.
Here only the Roll_No column can never contain an identical and NULL value. We know every student has a unique roll number. Therefore two students can never have the same roll number. This feature helps to identify each record in the database uniquely. Hence, we can make the Roll_No attribute a primary key.
Features of Primary Key
The following are the essential primary key features:
What is a Unique Key?
The unique key is a single column or combination of columns in a table to uniquely identify database records. A unique key prevents from storing duplicate values in the column. A table can contain multiple unique key columns, unlike a primary key column. This key is similar to the primary key, except that one NULL value can be stored in the unique key column. The unique key is also called unique constraints and can be referenced by another table's foreign key.
For example, let's consider the same table named students with attributes such as Stud_ID, Roll_No, Name, Mobile, and Email.
Here Stud_ID can be assigned as a unique constraint because each student must have a unique identification number. If a student changes the college, then he or she would not have any student ID. In that case, the entry may contain a NULL value because a unique key constraint allows storing NULL, but it should be only one.
Features of Unique key
The following are the essential unique key features:
Key differences between Primary and Unique Key
The following points explain the key differences between primary and candidate keys:
Primary Key vs. Unique Key Comparison Chart
The following comparison chart explains their main differences in a quick manner:
In this article, we have made a comparison between primary key and unique key constraints. Here we have concluded that a unique key is useful when we want the columns don't contain duplicate values. And the primary key is useful when we do not want to keep NULL value in the table. It can also be ideal when we have a foreign key in another table to create a relationship among tables.