One-to-one relationship design in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a table with a primary key.

SQL

CREATE TABLE Person (PersonID [1] PRIMARY KEY, Name VARCHAR(100));

Drag options to blanks, or click blank then click option'

AIDENTITY

BINT

CDATE

DVARCHAR

Attempts:

3 left

💡 Hint

Common Mistakes

Using VARCHAR instead of INT for the primary key.

Using DATE type for an ID column.

The column PersonID should be of type INT to store integer IDs.

2fill in blank

medium

Complete the code to create a table with a foreign key referencing PersonID.

SQL

CREATE TABLE Passport (PassportID INT PRIMARY KEY, PersonID INT [1] NULL, FOREIGN KEY (PersonID) REFERENCES Person(PersonID));

Drag options to blanks, or click blank then click option'

ANOT

BUNIQUE

CNULL

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the foreign key column nullable.

Using UNIQUE keyword incorrectly here.

The PersonID column should be NOT NULL to ensure every passport is linked to a person.

3fill in blank

hard

Fix the error in the code to enforce one-to-one relationship by adding a constraint.

SQL

ALTER TABLE Passport ADD CONSTRAINT [1] UNIQUE (PersonID);

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Afk_person

Bchk_personid

Cpk_passport

Duq_personid

Attempts:

3 left

💡 Hint

Common Mistakes

Using a foreign key name for a unique constraint.

Using a check constraint name instead of unique.

The UNIQUE constraint on PersonID ensures one-to-one relationship by preventing duplicates.

4fill in blank

hard

Fill both blanks to create a one-to-one relationship where PassportID is also the foreign key.

SQL

CREATE TABLE Passport (PassportID INT [1] PRIMARY KEY, FOREIGN KEY (PassportID) [2] REFERENCES Person(PersonID));

Drag options to blanks, or click blank then click option'

ANOT NULL

BUNIQUE

CON DELETE CASCADE

DON UPDATE CASCADE

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving PassportID nullable.

Not using ON DELETE CASCADE for dependent records.

PassportID must be NOT NULL and the foreign key uses ON DELETE CASCADE to remove passports if the person is deleted.

5fill in blank

hard

Fill all three blanks to insert a new person and their passport ensuring the one-to-one relationship.

SQL

INSERT INTO Person (PersonID, Name) VALUES ([1], 'Alice');
INSERT INTO Passport (PassportID, PersonID) VALUES ([2], [3]);

Drag options to blanks, or click blank then click option'

A1

B1001

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using different PersonID values in Passport.

Using same values for PassportID and PersonID.

PersonID is 1, PassportID is 1001, and Passport.PersonID matches PersonID 1 to keep one-to-one link.

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