ER diagram to table mapping in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL code to create a table for an entity named 'Student' with an ID as primary key.

SQL

CREATE TABLE Student (StudentID [1] PRIMARY KEY, Name VARCHAR(100));

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

AINT

BTEXT

CFLOAT

DDATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using TEXT or DATE for primary key which is not suitable for IDs.

The primary key should be an integer type for IDs in this case.

2fill in blank

medium

Complete the SQL code to create a table for a weak entity 'Dependent' with a foreign key referencing 'Employee'.

SQL

CREATE TABLE Dependent (DependentID INT, EmployeeID [1], PRIMARY KEY (DependentID, EmployeeID), FOREIGN KEY (EmployeeID) REFERENCES Employee(EmployeeID));

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

ADATE

BINT

CTEXT

DFLOAT

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different data type than the referenced primary key for foreign key.

The foreign key EmployeeID should have the same data type as the referenced primary key, which is INT.

3fill in blank

hard

Fix the error in the SQL code to correctly create a table for a many-to-many relationship 'Enrollment' between 'Student' and 'Course'.

SQL

CREATE TABLE Enrollment (StudentID INT, CourseID [1], PRIMARY KEY (StudentID, CourseID), FOREIGN KEY (StudentID) REFERENCES Student(StudentID), FOREIGN KEY (CourseID) REFERENCES Course(CourseID));

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

AFLOAT

BTEXT

CINT

DDATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using mismatched data types for foreign keys.

Both foreign keys should have the same data type as their referenced primary keys, which are INT.

4fill in blank

hard

Fill both blanks to create a table for an entity 'Book' with a composite primary key and a foreign key referencing 'Publisher'.

SQL

CREATE TABLE Book (ISBN [1], Edition [2], PRIMARY KEY (ISBN, Edition), PublisherID INT, FOREIGN KEY (PublisherID) REFERENCES Publisher(PublisherID));

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

AVARCHAR(13)

BINT

CDATE

DTEXT

Attempts:

3 left

💡 Hint

Common Mistakes

Using INT for ISBN or TEXT for Edition.

ISBN is a string of 13 characters, so VARCHAR(13) is used. Edition is a number, so INT is appropriate.

5fill in blank

hard

Fill all three blanks to create a table for an entity 'Employee' with an auto-increment primary key, a name, and a hire date.

SQL

CREATE TABLE Employee (EmployeeID [1] [2] PRIMARY KEY, Name [3] NOT NULL, HireDate DATE);

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

AINT

BVARCHAR(100)

CAUTO_INCREMENT

DTEXT

Attempts:

3 left

💡 Hint

Common Mistakes

Using TEXT for Name or missing AUTO_INCREMENT for EmployeeID.

EmployeeID is an INT with AUTO_INCREMENT for automatic numbering. Name is a VARCHAR(100) to store text.

Practice

1. In an ER diagram, what does an entity typically become when converting to a database schema?

easy

A. A table with columns for each attribute

B. A single column in a table

C. A database index

D. A stored procedure

Solution

1. Step 1: Understand what an entity represents

   An entity in an ER diagram represents a real-world object or concept with attributes.

2. Step 2: Map entity to database structure

   Each entity is converted into a table, where each attribute becomes a column in that table.

3. Final Answer:

   A table with columns for each attribute -> Option A

4. Quick Check:

   Entity = Table [OK]

Hint: Entities become tables with columns for attributes [OK]

Common Mistakes:

• Confusing entities with indexes
• Thinking entities become single columns
• Assuming entities become procedures

2. Which of the following is the correct way to represent a one-to-many relationship in tables derived from an ER diagram?

easy

A. Create a new table with only primary keys from both tables

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

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

D. Use a trigger to link the two tables

Solution

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

   One-to-many means one record in the first table relates to many records in the second table.

2. Step 2: Map relationship to tables

   The 'many' side table gets a foreign key column referencing the 'one' side table's primary key.

3. Final Answer:

   Add a foreign key column in the 'many' side table referencing the 'one' side -> Option B

4. Quick Check:

   Foreign key on 'many' side = Add a foreign key column in the 'many' side table referencing the 'one' side [OK]

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

Common Mistakes:

• Placing foreign key on the 'one' side
• Creating unnecessary tables for one-to-many
• Using triggers instead of foreign keys

3. Given two entities Author(id, name) and Book(id, title, author_id) with a one-to-many relationship from Author to Book, what will be the result of this SQL query?

SELECT a.name, b.title FROM Author a JOIN Book b ON a.id = b.author_id WHERE a.name = 'Alice';

medium

A. Syntax error due to join condition

B. List of all authors and their books

C. List of books with no authors

D. List of all books written by Alice

Solution

1. Step 1: Analyze the JOIN condition

   The query joins Author and Book on matching author IDs, linking books to their authors.

2. Step 2: Apply the WHERE filter

   It filters authors with name 'Alice', so only books by Alice are selected.

3. Final Answer:

   List of all books written by Alice -> Option D

4. Quick Check:

   Join + filter by author name = books by Alice [OK]

Hint: JOIN on foreign key filters books by author [OK]

Common Mistakes:

• Confusing join condition causing no results
• Ignoring WHERE clause filtering
• Thinking it lists all authors

4. You have two tables from an ER diagram: Student(id, name) and Enrollment(student_id, course_id). You want to add a foreign key constraint to Enrollment.student_id. Which SQL statement is correct?

medium

A. ALTER TABLE Enrollment ADD FOREIGN KEY (student_id) REFERENCES Student(id);

B. ALTER TABLE Student ADD FOREIGN KEY (id) REFERENCES Enrollment(student_id);

C. CREATE FOREIGN KEY Enrollment.student_id REFERENCES Student.id;

D. ALTER Enrollment ADD CONSTRAINT FOREIGN KEY student_id Student(id);

Solution

1. Step 1: Identify the correct syntax for adding foreign key

   The standard syntax is ALTER TABLE [table] ADD FOREIGN KEY (column) REFERENCES [other_table](column).

2. Step 2: Apply to given tables

   Enrollment.student_id references Student.id, so the statement must alter Enrollment table.

3. Final Answer:

   ALTER TABLE Enrollment ADD FOREIGN KEY (student_id) REFERENCES Student(id); -> Option A

4. Quick Check:

   ALTER TABLE + ADD FOREIGN KEY + REFERENCES [OK]

Hint: Foreign key added on referencing table with ALTER TABLE [OK]

Common Mistakes:

• Adding foreign key on referenced table
• Wrong ALTER TABLE syntax
• Using CREATE FOREIGN KEY instead of ALTER TABLE

5. Consider an ER diagram with entities Employee(emp_id, name), Project(proj_id, title), and a many-to-many relationship WorksOn between them. How should you map this relationship into tables?

hard

A. Add proj_id as a foreign key column in Employee table

B. Add emp_id as a foreign key column in Project table

C. Create a new table WorksOn with columns emp_id and proj_id as foreign keys

D. Merge Employee and Project tables into one

Solution

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

   Many-to-many means multiple employees can work on multiple projects and vice versa.

2. Step 2: Map many-to-many to tables

   This requires a new table (junction table) that holds foreign keys from both Employee and Project tables.

3. Final Answer:

   Create a new table WorksOn with columns emp_id and proj_id as foreign keys -> Option C

4. Quick Check:

   Many-to-many = junction table with two foreign keys [OK]

Hint: Many-to-many needs a new table with two foreign keys [OK]

Common Mistakes:

• Adding foreign key to only one table
• Merging unrelated tables
• Ignoring the need for a junction table