What is a primary key? Can a table have more than one primary key?

Difficulty: 4/10

primary key definition, composite primary key, primary key design

Understanding Primary Keys in MySQL

A primary key is a column or a combination of columns that uniquely identifies each row in a table. It ensures that no two rows have the same value in the primary key column(s) and that the value is never NULL.

1. Characteristics of a Primary Key

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Uniquely identifies each row in the table.
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Cannot contain NULL values.
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Automatically creates a unique index.
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Ensures entity integrity in relational databases.

2. Can a Table Have More Than One Primary Key?

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A table can only have one primary key constraint.
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However, that primary key can consist of multiple columns — this is called a composite primary key.
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You cannot define two separate primary keys in one table.

Example: Composite Primary Key

In this example, neither student_id nor course_id alone uniquely identifies a row, but the combination does — forming a composite primary key.

Scenario Questions

0-2 years experience

Follow-up Questions