A primary key is an attribute or a set of attributes that help to uniquely identify the tuples (records) in a relational table. It is a column or set of columns that helps to identify every record present in that table uniquely. A primary key is used as a unique identifier to quickly parse data within the table. It is a special relational database table column (or combination of columns) designated to uniquely identify each table record. Every entity in a data model should have a primary key, which should be based on a single attribute or a group of attributes.
Some key features of a primary key are:
- It must contain a unique value for each row of data.
- It cannot contain null values.
- Every row must have a primary key value.
A table cannot have more than one primary key. The choice of a primary key in a relational database often depends on the preference of the administrator. Its possible to change the primary key for a given database when the specific needs of the users change.
In summary, a primary key is a unique identifier for each record in a table, and it must contain a unique value for each row of data. It is a critical concept in relational databases and is necessary to ensure that a table record can always be uniquely identified.
