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What is a primary key?

Parthiban Kannan
Updated on November 4, 2025

1 min read

A primary key is a column or a set of columns in a database table that uniquely identifies each record in that table. It ensures that no two rows have the same value in the primary key column(s) and that the key is never NULL. Essentially, it enforces uniqueness and integrity for the table’s data.

Example:
In an Employees table, EmployeeID can be a primary key because each employee has a unique ID:

🗃️
filename.sql 
CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    Name VARCHAR(50),
    Department VARCHAR(50)
);

Here, EmployeeID ensures that no two employees can have the same ID, which helps maintain data accuracy.

Practical application:
I’ve used primary keys extensively while designing relational databases, especially when linking tables with foreign keys. For example, in a Sales table, CustomerID as a primary key in the Customers table allows me to reference customers in Sales without duplication.

Challenges faced:

  • One challenge is choosing the right primary key. Sometimes natural keys (like email) may seem unique, but they can change or have duplicates. That’s why I often use surrogate keys (like auto-increment IDs) to avoid these issues.
  • Another challenge arises when importing data—duplicates can cause primary key constraint errors, so pre-validation or cleaning is essential.

Limitation:

  • A table can have only one primary key, although it can consist of multiple columns (composite key).
  • Sometimes, a single-column surrogate key might not convey business meaning, so balancing uniqueness and meaning is important.

Alternative:

  • If you don’t want uniqueness strictly enforced, you can use a unique key constraint, which allows NULLs (except in some databases), unlike primary keys that cannot be NULL.
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