What is Primary Key in SQL ?
Primary Key is a constraint in SQL. So, before understanding what exactly is a primary key, let’s understand what exactly is a constraint in SQL. Constraints are the rules enforced on data columns on a table. These are used to limit the type of data that can go into a table. Constraints can either be column level or table level.
Let’s look at the different types of constraints which are present in SQL:
| Constraint | Description |
| NOT NULL | Ensures that a column cannot have a NULL value. |
| DEFAULT | Provides a default value for a column when none is specified. |
| UNIQUE | Ensures that all the values in a column are different |
| PRIMARY | Uniquely identifies each row/record in a database table |
| FOREIGN | Uniquely identifies a row/record in any another database table |
| CHECK | The CHECK constraint ensures that all values in a column satisfy certain conditions. |
| INDEX | Used to create and retrieve data from the database very quickly. |
You can consider Primary Key constraint to be a combination of UNIQUE and NOT NULL constraint. This means that if a column is set as a primary key, then this particular column cannot have any null values present in it and also all the values present in this column must be unique.
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Published on 14-Dec-2021 17:04:38