Converting ER diagrams to relational schema in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to represent an entity as a table with its primary key.

DBMS Theory

CREATE TABLE [1] (ID INT PRIMARY KEY);

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AAttribute

BRelation

CEntity

DTuple

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Relation' instead of the entity name as table name.

Confusing attribute names with table names.

In relational schema, an entity from an ER diagram is converted into a table named after the entity.

2fill in blank

medium

Complete the code to add a foreign key for a one-to-many relationship.

DBMS Theory

ALTER TABLE Orders ADD COLUMN [1] INT, ADD FOREIGN KEY ([1]) REFERENCES Customers(ID);

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ACustomerID

BOrderID

CProductID

DEmployeeID

Attempts:

3 left

💡 Hint

Common Mistakes

Using OrderID as foreign key in Orders table.

Using unrelated column names like ProductID.

In a one-to-many relationship, the foreign key is added to the 'many' side table referencing the 'one' side primary key.

3fill in blank

hard

Fix the error in the relational schema for a many-to-many relationship.

DBMS Theory

CREATE TABLE [1] (StudentID INT, CourseID INT, PRIMARY KEY (StudentID, CourseID));

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AEnrollment

BStudent

CStudentCourse

DCourse

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the table after only one entity.

Not including both foreign keys as primary key.

Many-to-many relationships are represented by a new table (often called Enrollment) with foreign keys from both related entities.

4fill in blank

hard

Fill both blanks to create a table for a weak entity with a composite primary key.

DBMS Theory

CREATE TABLE [1] (OwnerID INT, [2] INT, PRIMARY KEY (OwnerID, [2]));

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ADependent

BDependentID

CDependentName

DOwnerName

Attempts:

3 left

💡 Hint

Common Mistakes

Using attribute names that are not keys.

Not including both keys in the primary key.

Weak entities require a composite key including the owner's key and their own partial key.

5fill in blank

hard

Fill all three blanks to define a table with an attribute and a foreign key for a multivalued attribute.

DBMS Theory

CREATE TABLE [1] (EntityID INT, [2] VARCHAR(100), PRIMARY KEY (EntityID, [2]), FOREIGN KEY (EntityID) REFERENCES [3](ID));

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APhoneNumbers

BPhoneNumber

CEntity

DEntities

Attempts:

3 left

💡 Hint

Common Mistakes

Using the main entity table name for the multivalued attribute table.

Not including both keys in the primary key.

Multivalued attributes are represented by a separate table with a foreign key to the main entity and the attribute value.

Practice

1. In converting an ER diagram to a relational schema, how is a one-to-many relationship typically represented?

easy

A. Add a foreign key in the table on the 'many' side referencing the 'one' side

B. Create a new table for the relationship

C. Add a foreign key in the table on the 'one' side referencing the 'many' side

D. Merge both entities into a single table

Solution

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

   In a one-to-many relationship, one entity instance relates to many instances of another entity.

2. Step 2: Represent relationship in relational schema

   The foreign key is placed in the table representing the 'many' side, pointing to the 'one' side's primary key.

3. Final Answer:

   Add a foreign key in the table on the 'many' side referencing the 'one' side -> Option A

4. Quick Check:

   One-to-many = foreign key on many side [OK]

Hint: Foreign key goes on the 'many' side table [OK]

Common Mistakes:

• Placing foreign key on the 'one' side
• Creating unnecessary new tables for one-to-many
• Merging entities incorrectly

2. Which of the following is the correct way to represent a many-to-many relationship in a relational schema?

easy

A. Add foreign keys in both entity tables referencing each other

B. Merge both entities into a single table

C. Create a new table with foreign keys referencing both entities

D. Add a foreign key in the 'one' side table referencing the 'many' side

Solution

1. Step 1: Understand many-to-many relationship

   Many-to-many means multiple instances of one entity relate to multiple instances of another.

2. Step 2: Represent many-to-many in relational schema

   Create a new table (junction table) with foreign keys referencing the primary keys of both entities.

3. Final Answer:

   Create a new table with foreign keys referencing both entities -> Option C

4. Quick Check:

   Many-to-many = new junction table [OK]

Hint: Use a new table for many-to-many relationships [OK]

Common Mistakes:

• Adding foreign keys directly in entity tables
• Merging entities incorrectly
• Ignoring the need for a junction table

3. Given an ER diagram with entities Student(SID, Name) and Course(CID, Title) connected by a many-to-many relationship Enrolls, which relational schema correctly represents this?

medium

A. Student(SID, Name), Course(CID, Title, SID), Enrolls(SID, CID)

B. Student(SID, Name), Course(CID, Title), Enrolls(SID, CID)

C. Student(SID, Name, CID), Course(CID, Title)

D. Student(SID, Name), Course(CID, Title)

Solution

1. Step 1: Identify entities and relationship type

   Student and Course are entities; Enrolls is many-to-many between them.

2. Step 2: Convert entities and relationship to tables

   Student and Course become tables with their attributes. Enrolls becomes a new table with foreign keys SID and CID.

3. Final Answer:

   Student(SID, Name), Course(CID, Title), Enrolls(SID, CID) -> Option B

4. Quick Check:

   Many-to-many needs separate table [OK]

Hint: Many-to-many always needs a separate table [OK]

Common Mistakes:

• Adding foreign key in Course for Student
• Adding foreign key in Student for Course
• Omitting the Enrolls table

4. You converted an ER diagram with a one-to-one relationship between Person and Passport by adding a foreign key PersonID in Passport. However, the database allows multiple passports for one person. What is the likely mistake?

medium

A. Foreign key in Passport lacks a UNIQUE constraint

B. Foreign key should be in Person table, not Passport

C. Passport table should be merged with Person table

D. PersonID should be primary key in Passport table

Solution

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

   One-to-one means each person has at most one passport and vice versa.

2. Step 2: Check foreign key constraints

   Foreign key alone allows many passports per person unless a UNIQUE constraint is added to enforce one-to-one.

3. Final Answer:

   Foreign key in Passport lacks a UNIQUE constraint -> Option A

4. Quick Check:

   One-to-one needs UNIQUE foreign key [OK]

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

Common Mistakes:

• Assuming foreign key alone enforces one-to-one
• Placing foreign key in wrong table
• Merging tables unnecessarily

5. An ER diagram has entities Author(AID, Name), Book(BID, Title), and a many-to-many relationship Writes with an attribute RoyaltyPercentage. How should you convert this to a relational schema?

hard

A. Create Author(AID, Name), Book(BID, Title), Writes(AID, BID)

B. Create Author(AID, Name), Book(BID, Title, RoyaltyPercentage), Writes(AID, BID)

C. Create Author(AID, Name, RoyaltyPercentage), Book(BID, Title), Writes(AID, BID)

D. Create Author(AID, Name), Book(BID, Title), Writes(AID, BID, RoyaltyPercentage)

Solution

1. Step 1: Identify entities and relationship with attribute

   Author and Book are entities; Writes is many-to-many with attribute RoyaltyPercentage.

2. Step 2: Convert entities and relationship with attribute

   Entities become tables with their attributes. The many-to-many relationship becomes a new table including foreign keys and the attribute RoyaltyPercentage.

3. Final Answer:

   Create Author(AID, Name), Book(BID, Title), Writes(AID, BID, RoyaltyPercentage) -> Option D

4. Quick Check:

   Relationship attributes go in junction table [OK]

Hint: Put relationship attributes in the new junction table [OK]

Common Mistakes:

• Adding relationship attribute to entity tables
• Omitting the junction table
• Placing attribute in wrong table