ER diagram to table mapping in SQL - Mini Project: Build & Apply

ER Diagram to Table Mapping

📖 Scenario: You are designing a simple database for a library. The library keeps track of books and authors. Each book has a title and a publication year. Each author has a name and a country. A book can have one author, and an author can write many books.

🎯 Goal: Create SQL tables that represent the entities and relationships from the ER diagram of the library system.

📋 What You'll Learn

Create a table called Authors with columns AuthorID (primary key), Name, and Country.

Create a table called Books with columns BookID (primary key), Title, PublicationYear, and AuthorID (foreign key referencing Authors.AuthorID).

Use appropriate data types for each column.

Set AuthorID in Books as a foreign key to enforce the relationship.

💡 Why This Matters

🌍 Real World

Database designers often convert ER diagrams into SQL tables to build real databases for applications like libraries, stores, or social networks.

💼 Career

Understanding how to map ER diagrams to tables is essential for roles like database developer, data analyst, and backend engineer.

1

Create the Authors table

Write a SQL statement to create a table called Authors with columns AuthorID as an integer primary key, Name as text, and Country as text.

SQL

-- Create the Authors table with AuthorID, Name, and Country
-- Your code here

Hint

Use CREATE TABLE Authors and define AuthorID as INTEGER PRIMARY KEY.

2

Create the Books table

Write a SQL statement to create a table called Books with columns BookID as an integer primary key, Title as text, PublicationYear as integer, and AuthorID as integer.

SQL

CREATE TABLE Authors (
    AuthorID INTEGER PRIMARY KEY,
    Name TEXT,
    Country TEXT
);

-- Create the Books table with BookID, Title, PublicationYear, and AuthorID
-- Your code here

Hint

Define BookID as INTEGER PRIMARY KEY and include AuthorID as an integer column.

3

Add foreign key constraint

Modify the Books table creation to add a foreign key constraint on AuthorID referencing Authors(AuthorID).

SQL

CREATE TABLE Authors (
    AuthorID INTEGER PRIMARY KEY,
    Name TEXT,
    Country TEXT
);

-- Modify Books table to add foreign key constraint on AuthorID
CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT,
    PublicationYear INTEGER,
    AuthorID INTEGER,
    -- Your code here
);

Hint

Use FOREIGN KEY (AuthorID) REFERENCES Authors(AuthorID) inside the Books table definition.

4

Complete the table definitions

Ensure both Authors and Books tables are created with all columns and the foreign key constraint as specified.

SQL

-- Verify the complete table creation statements are present
-- Your code here

Hint

Check that both tables have all columns and the foreign key constraint.

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