One-to-one relationship design in SQL

Introduction

A one-to-one relationship links two pieces of information so each item in one table matches exactly one item in another table. This keeps data organized and avoids repeating details.

When you want to store extra details about a person separately, like a user and their profile.

When you want to split sensitive information into a different table for security.

When you want to keep optional information separate to save space.

When two tables share the same primary key but hold different types of data.

When you want to clearly separate data for easier updates and maintenance.

Syntax

SQL

CREATE TABLE TableA (
  id INT PRIMARY KEY,
  columnA datatype
);

CREATE TABLE TableB (
  id INT PRIMARY KEY,
  columnB datatype,
  FOREIGN KEY (id) REFERENCES TableA(id)
);

The primary key in TableB is also a foreign key referencing TableA's primary key.

This setup ensures each row in TableB matches exactly one row in TableA.

Examples

Each person has one passport. The passport's person_id matches the person's person_id.

SQL

CREATE TABLE Person (
  person_id INT PRIMARY KEY,
  name VARCHAR(100)
);

CREATE TABLE Passport (
  person_id INT PRIMARY KEY,
  passport_number VARCHAR(20),
  FOREIGN KEY (person_id) REFERENCES Person(person_id)
);

EmployeeDetails stores extra info for each employee, linked by employee_id.

SQL

CREATE TABLE Employee (
  employee_id INT PRIMARY KEY,
  name VARCHAR(100)
);

CREATE TABLE EmployeeDetails (
  employee_id INT PRIMARY KEY,
  address VARCHAR(200),
  phone VARCHAR(15),
  FOREIGN KEY (employee_id) REFERENCES Employee(employee_id)
);

Sample Program

This example creates two tables linked one-to-one by user_id. It inserts one user and their profile, then selects both together.

SQL

CREATE TABLE User (
  user_id INT PRIMARY KEY,
  username VARCHAR(50)
);

CREATE TABLE UserProfile (
  user_id INT PRIMARY KEY,
  bio TEXT,
  FOREIGN KEY (user_id) REFERENCES User(user_id)
);

INSERT INTO User (user_id, username) VALUES (1, 'alice');
INSERT INTO UserProfile (user_id, bio) VALUES (1, 'Loves hiking and reading.');

SELECT u.user_id, u.username, p.bio
FROM User u
JOIN UserProfile p ON u.user_id = p.user_id;

Important Notes

Make sure the foreign key column in the second table is also the primary key to enforce one-to-one.

You can also use UNIQUE constraints on the foreign key if the primary key is different.

One-to-one relationships help keep data clean and avoid duplication.

Summary

One-to-one relationships link exactly one row in one table to one row in another.

Use the same primary key in both tables or a unique foreign key to enforce this.

This design helps organize data and separate concerns.

Practice

1. What is a key characteristic of a one-to-one relationship in database design?

easy

A. Each row in one table matches exactly one row in another table

B. Each row in one table can match many rows in another table

C. Rows in both tables have no connection

D. One table contains all data without links

Solution

1. Step 1: Understand one-to-one relationship meaning

   A one-to-one relationship means each record in one table corresponds to exactly one record in another table.

2. Step 2: Compare options to definition

   Each row in one table matches exactly one row in another table matches this definition perfectly, while others describe different relationships or no relationship.

3. Final Answer:

   Each row in one table matches exactly one row in another table -> Option A

4. Quick Check:

   One-to-one = single matching row [OK]

Hint: One-to-one means one row links to exactly one row [OK]

Common Mistakes:

• Confusing one-to-one with one-to-many
• Thinking tables have no relation
• Assuming one table holds all data

2. Which SQL constraint is commonly used to enforce a one-to-one relationship between two tables?

easy

A. FOREIGN KEY without UNIQUE

B. CHECK constraint on any column

C. UNIQUE constraint on the foreign key column

D. NOT NULL constraint on primary key

Solution

1. Step 1: Identify constraint enforcing uniqueness

   To ensure one-to-one, the foreign key must be unique so no duplicates link to the same row.

2. Step 2: Match constraints to this need

   UNIQUE constraint on the foreign key column enforces this, while FOREIGN KEY alone does not guarantee uniqueness.

3. Final Answer:

   UNIQUE constraint on the foreign key column -> Option C

4. Quick Check:

   Unique foreign key = one-to-one [OK]

Hint: Use UNIQUE on foreign key to enforce one-to-one [OK]

Common Mistakes:

• Using FOREIGN KEY without UNIQUE allows many-to-one
• Confusing CHECK with uniqueness
• Assuming NOT NULL enforces one-to-one

3. Given these tables:

CREATE TABLE Person (
  PersonID INT PRIMARY KEY,
  Name VARCHAR(50)
);

CREATE TABLE Passport (
  PassportID INT PRIMARY KEY,
  PersonID INT UNIQUE,
  Number VARCHAR(20),
  FOREIGN KEY (PersonID) REFERENCES Person(PersonID)
);

What does the UNIQUE constraint on PersonID in Passport ensure?

medium

A. Each person can have multiple passports

B. Each passport belongs to exactly one person, and each person has at most one passport

C. PersonID can be null in Passport

D. PassportID can be duplicated

Solution

1. Step 1: Understand UNIQUE on PersonID in Passport

   The UNIQUE constraint means no two rows in Passport can have the same PersonID, so one person links to at most one passport.

2. Step 2: Analyze relationship enforced

   Since Passport has a foreign key to Person and PersonID is unique, each passport belongs to one person, and each person can have only one passport.

3. Final Answer:

   Each passport belongs to exactly one person, and each person has at most one passport -> Option B

4. Quick Check:

   Unique foreign key = one-to-one link [OK]

Hint: UNIQUE foreign key means one-to-one link [OK]

Common Mistakes:

• Thinking one person can have many passports
• Ignoring UNIQUE constraint effect
• Assuming null allowed without checking

4. Consider this table design:

CREATE TABLE Employee (
  EmployeeID INT PRIMARY KEY,
  Name VARCHAR(50)
);

CREATE TABLE EmployeeDetails (
  DetailID INT PRIMARY KEY,
  EmployeeID INT,
  Address VARCHAR(100),
  FOREIGN KEY (EmployeeID) REFERENCES Employee(EmployeeID)
);

What is missing to enforce a one-to-one relationship between Employee and EmployeeDetails?

medium

A. Add PRIMARY KEY on EmployeeID in EmployeeDetails

B. Add NOT NULL constraint on EmployeeID in EmployeeDetails

C. Remove FOREIGN KEY constraint

D. Add UNIQUE constraint on EmployeeID in EmployeeDetails

Solution

1. Step 1: Identify current constraints

   EmployeeDetails has a foreign key to Employee but no uniqueness on EmployeeID, so multiple details can link to one employee.

2. Step 2: Determine what enforces one-to-one

   Adding UNIQUE on EmployeeID ensures each employee links to at most one detail, enforcing one-to-one.

3. Final Answer:

   Add UNIQUE constraint on EmployeeID in EmployeeDetails -> Option D

4. Quick Check:

   Unique foreign key needed for one-to-one [OK]

Hint: Add UNIQUE on foreign key column for one-to-one [OK]

Common Mistakes:

• Assuming NOT NULL enforces one-to-one
• Removing foreign key breaks relationship
• Confusing primary key with foreign key uniqueness

5. You want to split user data into two tables: User and UserProfile. Each user has exactly one profile. Which design best enforces this one-to-one relationship?

hard

A. User and UserProfile share the same primary key column

B. UserProfile has a foreign key to User without UNIQUE constraint

C. UserProfile has no foreign key but a separate primary key

D. UserProfile has a foreign key to User with UNIQUE constraint on that foreign key

Solution

1. Step 1: Understand one-to-one enforcement methods

   One way is to share the same primary key in both tables, ensuring exactly one matching row.

2. Step 2: Compare options

   User and UserProfile share the same primary key column uses the same primary key in both tables, which is a strong one-to-one design. UserProfile has a foreign key to User with UNIQUE constraint on that foreign key is valid but less strict. Options B and C do not enforce one-to-one properly.

3. Final Answer:

   User and UserProfile share the same primary key column -> Option A

4. Quick Check:

   Shared primary key = strict one-to-one [OK]

Hint: Use shared primary key for strict one-to-one [OK]

Common Mistakes:

• Ignoring uniqueness on foreign key
• Assuming foreign key alone enforces one-to-one
• Not linking tables properly