One-to-one relationship design in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a one-to-one relationship in database design?

A one-to-one relationship means each record in one table matches exactly one record in another table, like a person having one passport.

intermediate

How do you enforce a one-to-one relationship between two tables in SQL?

Use a unique foreign key in one table that references the primary key of the other table, ensuring each key appears only once.

intermediate

Why might you split data into two tables with a one-to-one relationship?

To separate optional or sensitive data, improve organization, or optimize performance by loading only needed data.

beginner

Which SQL constraint ensures that a foreign key is unique in a one-to-one relationship?

The UNIQUE constraint on the foreign key column ensures no duplicates, enforcing one-to-one mapping.

intermediate

Example: How to create two tables with a one-to-one relationship between 'Person' and 'Passport'?

Create 'Person' with primary key 'person_id'. Create 'Passport' with 'passport_id' as primary key and 'person_id' as unique foreign key referencing 'Person'.

What does a one-to-one relationship mean in databases?

A Many records in one table match many records in another table

B One record in a table matches many records in another table

C Each record in one table matches exactly one record in another table

D No relationship between tables

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

Which SQL constraint is essential to enforce a one-to-one relationship?

A UNIQUE constraint on the foreign key

B DEFAULT constraint

C CHECK constraint

D NOT NULL constraint

The UNIQUE constraint on the foreign key column ensures each foreign key value appears only once, enforcing one-to-one.

In a one-to-one relationship, where is the foreign key usually placed?

A In the table with optional or additional data

B In both tables

C In the table with the primary key only

D Nowhere, foreign keys are not used

The foreign key is placed in the table holding optional or extra data to link back to the main table.

Why split data into two tables with a one-to-one relationship?

A To duplicate data

B To avoid using foreign keys

C To make queries slower

D To separate optional or sensitive data

Splitting helps organize data, keep sensitive info separate, and optimize performance.

Which SQL statement correctly creates a one-to-one relationship?

A CREATE TABLE Passport (passport_id INT PRIMARY KEY, person_id INT, FOREIGN KEY (person_id) REFERENCES Person(person_id));

B CREATE TABLE Passport (passport_id INT PRIMARY KEY, person_id INT UNIQUE, FOREIGN KEY (person_id) REFERENCES Person(person_id));

C CREATE TABLE Passport (passport_id INT PRIMARY KEY, person_id INT);

D CREATE TABLE Passport (passport_id INT PRIMARY KEY, person_id INT NOT NULL);

Option B adds UNIQUE to person_id foreign key, enforcing one-to-one relationship.

Explain how to design a one-to-one relationship between two tables in SQL.

Think about keys and uniqueness to link tables.

Why would you use a one-to-one relationship instead of combining all data in one table?

Consider reasons for splitting data logically.

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