Division operation in DBMS Theory - Mini Project: Build & Apply

Division Operation in SQL

📖 Scenario: You work in a library database. You want to find authors who have written books in all genres available in the library.

🎯 Goal: Build an SQL query using the division operation concept to find authors who have written books in every genre listed.

📋 What You'll Learn

Create a table called AuthorsBooks with columns author and genre.

Create a table called Genres with a single column genre listing all genres.

Write a query that finds authors who have written books in all genres using division operation logic.

Use NOT EXISTS and EXCEPT or equivalent to implement the division operation.

💡 Why This Matters

🌍 Real World

Finding entities that relate to all items in another set is common in business and data analysis, such as customers who bought all products or employees trained in all required skills.

💼 Career

Understanding division operation helps in writing complex SQL queries for data filtering and reporting, a valuable skill for data analysts, database administrators, and backend developers.

1

Create the AuthorsBooks table with sample data

Create a table called AuthorsBooks with columns author and genre. Insert these exact rows: ('Alice', 'Fiction'), ('Alice', 'History'), ('Bob', 'Fiction'), ('Bob', 'Science'), ('Carol', 'Fiction'), ('Carol', 'History'), ('Carol', 'Science').

DBMS Theory

-- Create AuthorsBooks table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Create the Genres table with all genres

Create a table called Genres with a single column genre. Insert these exact genres: 'Fiction', 'History', 'Science'.

DBMS Theory

CREATE TABLE AuthorsBooks (author VARCHAR(50), genre VARCHAR(50));
INSERT INTO AuthorsBooks (author, genre) VALUES
('Alice', 'Fiction'),
('Alice', 'History'),
('Bob', 'Fiction'),
('Bob', 'Science'),
('Carol', 'Fiction'),
('Carol', 'History'),
('Carol', 'Science');
-- Create Genres table and insert data
-- Your code here

Hint

Use CREATE TABLE and INSERT INTO to add the genres.

3

Write the division operation query to find authors with all genres

Write a query using NOT EXISTS and EXCEPT to find authors who have written books in all genres listed in the Genres table. Use author and genre columns from AuthorsBooks.

DBMS Theory

CREATE TABLE AuthorsBooks (author VARCHAR(50), genre VARCHAR(50));
INSERT INTO AuthorsBooks (author, genre) VALUES
('Alice', 'Fiction'),
('Alice', 'History'),
('Bob', 'Fiction'),
('Bob', 'Science'),
('Carol', 'Fiction'),
('Carol', 'History'),
('Carol', 'Science');

CREATE TABLE Genres (genre VARCHAR(50));
INSERT INTO Genres (genre) VALUES
('Fiction'),
('History'),
('Science');

-- Write the division operation query below
-- Your code here

Hint

Use NOT EXISTS with a subquery that uses EXCEPT to find missing genres per author.

4

Complete the query with ordering and final touches

Add an ORDER BY author clause at the end of the query to list authors alphabetically.

DBMS Theory

CREATE TABLE AuthorsBooks (author VARCHAR(50), genre VARCHAR(50));
INSERT INTO AuthorsBooks (author, genre) VALUES
('Alice', 'Fiction'),
('Alice', 'History'),
('Bob', 'Fiction'),
('Bob', 'Science'),
('Carol', 'Fiction'),
('Carol', 'History'),
('Carol', 'Science');

CREATE TABLE Genres (genre VARCHAR(50));
INSERT INTO Genres (genre) VALUES
('Fiction'),
('History'),
('Science');

SELECT DISTINCT author
FROM AuthorsBooks A
WHERE NOT EXISTS (
  SELECT genre FROM Genres
  EXCEPT
  SELECT genre FROM AuthorsBooks WHERE author = A.author
);
-- Add ORDER BY clause below
-- Your code here

Hint

Use ORDER BY author to sort the results alphabetically.

Practice

1.

What does the division operation in a database primarily help to find?

easy

A. Entries that appear only once in a table

B. The sum of values in a column

C. The total count of rows in a table

D. Entries in one table related to all entries in another table

Solution

1. Step 1: Understand the division operation purpose

   The division operation is used to find records in one table that are related to every record in another table.

2. Step 2: Compare with other options

   The other options describe operations like finding unique entries, counting rows, or summing values, not the division operation.

3. Final Answer:

   Entries in one table related to all entries in another table -> Option D

4. Quick Check:

   Division operation = find all related entries [OK]

Hint: Division finds entries matching all in another set [OK]

Common Mistakes:

• Confusing division with counting or summing
• Thinking it finds entries related to some, not all
• Mixing division with join operations

2.

Which SQL keyword is commonly used to implement the division operation?

easy

A. GROUP BY

B. NOT EXISTS

C. HAVING

D. EXISTS

Solution

1. Step 1: Recall division implementation in SQL

   Division is often implemented using NOT EXISTS to check that no unmatched entries exist.

2. Step 2: Analyze other keywords

   GROUP BY groups rows, HAVING filters groups, and EXISTS checks existence but not for division logic.

3. Final Answer:

   NOT EXISTS -> Option B

4. Quick Check:

   Division uses NOT EXISTS for completeness check [OK]

Hint: Use NOT EXISTS to ensure all related entries exist [OK]

Common Mistakes:

• Using EXISTS instead of NOT EXISTS
• Confusing GROUP BY with division logic
• Assuming HAVING filters individual rows

3.

Consider two tables: Suppliers(supplier_id) and Parts(part_id), and a relation Supplies(supplier_id, part_id). Which SQL query finds suppliers who supply all parts?

SELECT supplier_id FROM Suppliers S WHERE NOT EXISTS (
  SELECT part_id FROM Parts P WHERE NOT EXISTS (
    SELECT * FROM Supplies SP WHERE SP.supplier_id = S.supplier_id AND SP.part_id = P.part_id
  )
);

medium

A. Finds parts not supplied by any supplier

B. Finds suppliers supplying no parts

C. Finds suppliers supplying all parts

D. Finds parts supplied by all suppliers

Solution

1. Step 1: Understand the nested NOT EXISTS logic

   The inner NOT EXISTS checks if a supplier does not supply a part. The outer NOT EXISTS ensures no such part exists, meaning the supplier supplies all parts.

2. Step 2: Match query purpose with options

   This query finds suppliers who supply every part, matching Finds suppliers supplying all parts.

3. Final Answer:

   Finds suppliers supplying all parts -> Option C

4. Quick Check:

   Double NOT EXISTS = all parts supplied [OK]

Hint: Double NOT EXISTS means 'for all' condition [OK]

Common Mistakes:

• Misreading NOT EXISTS as EXISTS
• Confusing suppliers with parts
• Ignoring nested query logic

4.

Identify the error in this SQL query intended to find students who have completed all courses:

SELECT student_id FROM Students S WHERE EXISTS (
  SELECT course_id FROM Courses C WHERE NOT EXISTS (
    SELECT * FROM Completed WHERE student_id = S.student_id AND course_id = C.course_id
  )
);

medium

A. Should use NOT EXISTS instead of EXISTS in outer query

B. Missing GROUP BY clause

C. Incorrect table alias usage

D. Should use INNER JOIN instead of subqueries

Solution

1. Step 1: Analyze the logic of EXISTS and NOT EXISTS

   The inner NOT EXISTS checks if a student hasn't completed a course. The outer EXISTS checks if such a course exists, meaning the student missed some courses.

2. Step 2: Correct the logic for 'all courses completed'

   To find students who completed all courses, the outer condition should be NOT EXISTS to ensure no missing courses.

3. Final Answer:

   Should use NOT EXISTS instead of EXISTS in outer query -> Option A

4. Quick Check:

   Use NOT EXISTS outside to confirm completeness [OK]

Hint: Outer NOT EXISTS ensures no missing entries [OK]

Common Mistakes:

• Using EXISTS instead of NOT EXISTS outside
• Assuming GROUP BY is needed here
• Confusing alias names

5.

You have two tables: Employees(emp_id) and Projects(proj_id), and a relation WorksOn(emp_id, proj_id). How would you find employees who work on every project that a specific manager manages, given a table Manages(manager_id, proj_id)?

hard

A. Use division operation with WorksOn and projects managed by the manager

B. Use a simple JOIN between Employees and Projects

C. Use GROUP BY on WorksOn without filtering projects

D. Use EXISTS to find employees working on any project

Solution

1. Step 1: Identify the problem as a division operation

   We want employees who work on all projects managed by a specific manager, which is a classic division problem.

2. Step 2: Apply division using WorksOn and filtered projects

   Filter projects by the manager in Manages, then find employees whose WorksOn covers all those projects.

3. Final Answer:

   Use division operation with WorksOn and projects managed by the manager -> Option A

4. Quick Check:

   Division finds employees working on all manager's projects [OK]

Hint: Filter projects by manager, then apply division on WorksOn [OK]

Common Mistakes:

• Ignoring filtering projects by manager
• Using JOIN without division logic
• Assuming GROUP BY alone solves the problem