Understand the purpose of a primary key and identify one

What is a Primary Key?

A primary key is a special column (or set of columns) in a database table that uniquely identifies each row. Think of it as a unique ID card for every record.

• It must be unique – no two rows can have the same primary key value.
• It cannot be NULL – every row must have a value.
• It is often used as a reference in other tables (foreign keys).

🔑 Example: In a table of students, the student_id column could be the primary key.

Analogy: Library Books

Imagine a library where each book has a unique ISBN. Even if two books have the same title, their ISBNs are different, so you can always find the exact book you want. The ISBN is like a primary key for books.

Identifying a Primary Key in a Table

When you look at a table definition, the primary key is usually highlighted or marked with PRIMARY KEY. If it’s not obvious, look for a column that:

1. Has a unique value for every row.
2. Is not nullable.
3. Often has a name like id, student_id, or order_id.

Composite Primary Key

Sometimes a single column isn’t enough to guarantee uniqueness. In that case, two or more columns together form a primary key.

📌 Example: In a table that records student_id and course_id for enrolments, the combination of both columns is unique.

Exam Tips

• Look for a column marked PRIMARY KEY or named id.
• Check that the column is unique and not null.
• Remember that a composite key is a combination of two or more columns.
• When answering “Identify the primary key”, write the column name(s) exactly as shown in the table.