What are primary keys?

Primary keys are a fundamental concept in relational databases, serving several key functions:

Definition

A primary key is a column (or a combination of columns) in a database table that uniquely identifies each record in that table.

Key Characteristics

1. Uniqueness: Each value in a primary key column must be unique across all records in the table. This ensures that no two rows can have the same primary key value.

2. Non-nullability: A primary key cannot contain NULL values. Every record must have a valid primary key value.

3. Single Identifier: Typically, a primary key is a single column, but it can also be a composite key made up of multiple columns.

Purpose

• Record Identification: Primary keys provide a way to uniquely identify each record, making it easier to retrieve, update, or delete specific entries.
• Data Integrity: They help maintain the integrity of the data by preventing duplicate entries and ensuring that each record can be reliably referenced.
• Relationships: Primary keys are often used in conjunction with foreign keys to establish relationships between tables, facilitating data organization and retrieval.

Example

Consider a users table:

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100) UNIQUE
);

In this example:

• user_id is the primary key, uniquely identifying each user.
• It cannot be NULL and must be unique for every user in the table.

In summary, primary keys are essential for ensuring that each record in a database table is unique and can be accurately referenced, which is crucial for maintaining the overall integrity of the database. If you have more questions or need further clarification, feel free to ask!