Summary  
A primary key is a table constraint that enforces uniqueness on one or more columns, providing each record with a unique identifier to ensure data integrity and enable efficient record access.  

General domain of usage  
Relational database schema design

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:

```python
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.

Which of the following statements best describes a `PRIMARY KEY` in a database?

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