Primary Keys, Foreign Keys & Relationships Explained

Primary Keys, Foreign Keys & Relationships

1 min readPublished 22 Jul 2026

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Keys are what make a database relational — they uniquely identify rows and link tables together.

Interactive explainer

The Relational Model

Tables, rows, columns and keys — the foundation of SQL.

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Tables

One kind of thing.

A relational database stores data in tables (relations). Each table holds one kind of entity — Customers, Orders, Products — like a well-defined spreadsheet.

Check your understanding

1. What makes a database 'relational'?

2. What kind of language is SQL?

Keep learning

Databases & SQL Fundamentals Keys & Relationships Microsoft DP-900

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The essentials

A primary key uniquely identifies each row (e.g. customer_id) — unique and never null.
A foreign key references a primary key in another table, creating a relationship and enforcing referential integrity (you can't reference a customer that doesn't exist).
Relationship types: one-to-one, one-to-many (the most common — one customer, many orders) and many-to-many (resolved with a junction table).

Keys are the backbone of JOINs and good design.

Interview Intelligence

How this topic actually shows up in interviews — and how to demonstrate you understand it.

Why employers ask about this

Keys and relationships are fundamental; getting them wrong corrupts data integrity.

Technical questions

Explain primary vs foreign keys and referential integrity.+

PK uniquely identifies a row; FK links to a PK elsewhere; referential integrity stops orphaned references.

Behavioural questions

Tell me about catching a data-integrity issue.+

Show spotting the cause (missing constraint) and fixing it properly.

Real-world scenarios

“You need to link students and courses.”+

Expected answer: That's many-to-many — create a junction table (enrolments) with foreign keys to both.

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Employability Intelligence

Where this knowledge takes you — the jobs, skills and certifications it feeds into.

Relevant roles

Data AnalystBackend DeveloperDatabase Administrator

Skills you're proving

KeysReferential integrityRelationships

Recommended certifications

Microsoft DP-900 (Data Fundamentals)Oracle / SQL certifications

Career progression

Databases & SQL → developer/DBA roles.

What employers expect

That you can design a sensible schema and write correct, efficient SQL.

Frequently asked questions

What is a primary key?

A column (or set of columns) that uniquely identifies each row — unique and non-null.

What is a foreign key?

A column that references a primary key in another table, creating a relationship and enforcing referential integrity.

How do you model a many-to-many relationship?

With a junction (bridge) table holding foreign keys to both related tables.

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