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What is the primary key?

The primary key is the column in the table that uniquely identifies each tuple (row) in the table. The primary key enforces integrity constraints on the table. Only one primary key is allowed in the table. The primary key does not accept any duplicate or null values. The primary key values in the table rarely change, so you need to be careful when choosing the primary key and choose places that rarely change. The primary key of one table can be referenced by the foreign key of another table.

To better understand the primary key, we create a table called Student, which has attributes such as roll_number, name, batch, phone_number, citizen_id, and so on.

In the above example, the roll_number attribute can never have the same null value, because students enrolled in each university have a unique Roll_number, so two students cannot have the same Roll_number, and each row in the table can be uniquely identified by the student's roll_number attribute. Therefore, in this case, we can use the Roll_number attribute as the primary key.

What is the only key?

Unique key constraints uniquely identify a single tuple in a relationship or table. Unlike primary keys, a table can have multiple unique keys. A unique key constraint can accept only one null value for a column; the unique constraint is also referenced by the foreign key of another table. It can be used when someone wants to impose unique constraints on columns and groups of columns that are not primary keys.

To better understand the unique key, we use the Student table with the Roll_number,Name,Batch,Phone_number and Citizen_ID attributes; where the Roll_number attribute has been given to the primary key.

In this example, a unique constraint can be assigned to the Citizen_ID, where each entry in the Citizen_ID column should be unique, not duplicate, because each citizen of a country must have its unique identification number. However, if a student migrates from another country, in this case he or she will not have a Citizen_ID, and the entry may have a null value because a NULL is allowed in a unique constraint.

The main differences between primary keys and unique keys:

1. When an attribute is declared as a primary key, it will not accept null values. On the other hand, when declared as a property of Unique, it can accept a null value.

2. There can be only one primary key in the table, but there can be multiple unique keys.

3. A clustered index is automatically created when a primary key is defined. Instead, the Unique key generates a nonclustered index.

That's all about the difference between the Mysql primary key and the unique key. Thank you for your support.

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