A PRIMARY KEY is a constraint applied to one or more columns within a table to ensure that each record in the table is uniquely identifiable. It serves as a unique identifier for each row, making it easy to reference and manipulate specific records.

Here's how to define a PRIMARY KEY in the structure of a table:

create_table_query = '''
    CREATE TABLE users (
        id INTEGER PRIMARY KEY,
        username TEXT,
        email TEXT
    )
'''

The "id" column is defined as the PRIMARY KEY. This means that each record in the "users" table will have a unique "id" value, which can be used to identify and access specific records. A PRIMARY KEY is crucial for ensuring data integrity and enabling efficient data retrieval.