One-to-One Relationship

Introduction

A 1:1 relationship means each row in table A maps to exactly one row in table B. Common when you split rarely-used or sensitive columns into a sister table.

Understanding the topic

Core concepts to understand:

• Implementation: profiles.user_id as both FK and UNIQUE.
• Use cases: separating PII, optional details, or frequently-vs-rarely accessed data.
• Could you put it in one table? Often yes — split when it earns its keep.
• Performance: smaller hot table; cold data lives separately.
• Cascade delete keeps the pair consistent.

Syntax reference

Visual workflow / architecture:

users               profiles
┌────┐ 1:1  UNIQUE   ┌──────────┐
│ id │ ─────────────▶│ user_id  │ (UNIQUE FK)
└────┘               │ bio, dob │
                     └──────────┘

Real-world use

Many SaaS apps split users (auth) from profiles (PII), so security policies can differ.

Best practices

• Add UNIQUE on the FK in B to enforce 1:1.
• Consider whether merging into one table is simpler.
• Use CASCADE delete to keep the pair in sync.

Hands-on exercise

Interview preparation — practice these questions:

• Q1. How do you enforce 1:1 in SQL?
• Q2. When would you split a 1:1 vs keep in one table?
• Q3. Do you need a FK for 1:1?
• Q4. CASCADE behavior in 1:1.
• Q5. Is 1:1 common in practice?