A primary key in a database is a unique identifier that is used to identify each row or record in a table. It is a special type of constraint that ensures that each row in a table has a unique identifier and that the identifier cannot be null or empty.
In most cases, a primary key is a single column in the table, although it can be a combination of columns if necessary. The primary key must be unique across all rows in the table and cannot be duplicated. It is also used to establish relationships between tables through foreign keys.
Primary keys play an important role in maintaining the integrity and consistency of the data in a database. They ensure that there are no duplicate rows or conflicting data and that each record can be easily and uniquely identified. They also facilitate the querying and reporting of data, as queries often rely on primary keys to identify and join data from multiple tables.