What is a primary key?

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

🔑 Key Characteristics of a Primary Key:

1. Uniqueness: Each value must be unique — no two rows can have the same primary key value.
2. Not Null: A primary key column cannot contain NULL values.
3. One per table: A table can have only one primary key, but it can consist of multiple columns (called a composite key).

Example:

CREATE TABLE students (
    student_id INT PRIMARY KEY,
    name VARCHAR(100),
    email VARCHAR(100)
);

Here:

• student_id is the primary key.
• Each student must have a unique student_id.
• student_id cannot be NULL.

Composite Primary Key Example:

CREATE TABLE enrollments (
    student_id INT,
    course_id INT,
    PRIMARY KEY (student_id, course_id)
);

Here:

• The combination of student_id and course_id must be unique.
• This is useful when a single column isn’t enough to uniquely identify a row.

Why Use a Primary Key?

• Ensures data integrity (no duplicate or null values).
• Helps in establishing relationships with other tables (via foreign keys).
• Improves query performance through indexing (most databases automatically index the primary key).

If you want help designing a primary key for a specific use case, I can help with that too!