ER diagram to table mapping in SQL

Introduction

We use ER diagrams to plan how data is organized. Mapping them to tables helps us create the actual database structure.

When designing a new database for a small business.

When converting a paper design of data relationships into a working database.

When explaining database structure to team members using visuals and then building it.

When updating an existing database to match new data requirements.

When learning how to organize data logically before coding.

Syntax

SQL

-- Basic steps to map ER diagram elements to tables
-- 1. Each entity becomes a table
-- 2. Each attribute becomes a column
-- 3. Primary keys become primary keys in tables
-- 4. Relationships become foreign keys or new tables

CREATE TABLE EntityName (
  PrimaryKeyColumn datatype PRIMARY KEY,
  Attribute1 datatype,
  Attribute2 datatype,
  ...
);

-- For many-to-many relationships, create a new table with foreign keys
CREATE TABLE RelationshipName (
  Entity1_ID datatype,
  Entity2_ID datatype,
  PRIMARY KEY (Entity1_ID, Entity2_ID),
  FOREIGN KEY (Entity1_ID) REFERENCES Entity1(PrimaryKey),
  FOREIGN KEY (Entity2_ID) REFERENCES Entity2(PrimaryKey)
);

Each entity in the ER diagram becomes a table in the database.

Relationships can be represented by foreign keys or separate tables depending on their type.

Examples

This creates a table for the 'Student' entity with ID, name, and age.

SQL

CREATE TABLE Student (
  StudentID INT PRIMARY KEY,
  Name VARCHAR(100),
  Age INT
);

This creates a table for the 'Course' entity.

SQL

CREATE TABLE Course (
  CourseID INT PRIMARY KEY,
  Title VARCHAR(100),
  Credits INT
);

This table represents a many-to-many relationship between students and courses.

SQL

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

Sample Program

This example maps two entities: Author and Book. Each author has many books, so Book has a foreign key AuthorID.

SQL

CREATE TABLE Author (
  AuthorID INT PRIMARY KEY,
  Name VARCHAR(100)
);

CREATE TABLE Book (
  BookID INT PRIMARY KEY,
  Title VARCHAR(100),
  AuthorID INT,
  FOREIGN KEY (AuthorID) REFERENCES Author(AuthorID)
);

Important Notes

Always define primary keys to uniquely identify each row.

Use foreign keys to link related tables and keep data consistent.

Many-to-many relationships need a separate table with foreign keys from both entities.

Summary

ER diagrams help plan database tables and their relationships.

Entities become tables, attributes become columns.

Relationships become foreign keys or new tables depending on type.

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