DML (INSERT, UPDATE, DELETE) in DBMS Theory - Mini Project: Build & Apply

DML (INSERT, UPDATE, DELETE) Basics

📖 Scenario: You are managing a small library database. You need to add new books, update book details, and remove old books from the database.

🎯 Goal: Build a set of SQL commands to insert new records, update existing records, and delete records from a books table.

📋 What You'll Learn

Create an initial books table with columns id, title, and author

Insert three specific book records into the books table

Update the author of a specific book by id

Delete a book record by id

💡 Why This Matters

🌍 Real World

Managing data in databases is essential for many applications like libraries, stores, and websites. Knowing how to add, change, and remove data helps keep information accurate and up to date.

💼 Career

Database administrators, backend developers, and data analysts use DML commands daily to maintain and manipulate data stored in databases.

1

Create the books table

Write a SQL statement to create a table called books with three columns: id as an integer primary key, title as text, and author as text.

DBMS Theory

-- Write the CREATE TABLE statement for books
-- Your code here

Hint

Use CREATE TABLE books (id INTEGER PRIMARY KEY, title TEXT, author TEXT); to define the table.

2

Insert three books into books

Write three separate SQL INSERT INTO books statements to add these books exactly:
1. id=1, title='1984', author='George Orwell'
2. id=2, title='To Kill a Mockingbird', author='Harper Lee'
3. id=3, title='The Great Gatsby', author='F. Scott Fitzgerald'

DBMS Theory

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    author TEXT
);
-- Insert the three books below
-- Your code here

Hint

Use INSERT INTO books (id, title, author) VALUES (...); for each book.

3

Update the author of the book with id=3

Write a SQL UPDATE statement to change the author of the book with id=3 to 'Francis Scott Fitzgerald'.

DBMS Theory

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    author TEXT
);

INSERT INTO books (id, title, author) VALUES (1, '1984', 'George Orwell');
INSERT INTO books (id, title, author) VALUES (2, 'To Kill a Mockingbird', 'Harper Lee');
INSERT INTO books (id, title, author) VALUES (3, 'The Great Gatsby', 'F. Scott Fitzgerald');

-- Update the author of the book with id=3
-- Your code here

Hint

Use UPDATE books SET author = 'Francis Scott Fitzgerald' WHERE id = 3; to update the author.

4

Delete the book with id=2

Write a SQL DELETE FROM books statement to remove the book record where id=2.

DBMS Theory

CREATE TABLE books (
    id INTEGER PRIMARY KEY,
    title TEXT,
    author TEXT
);

INSERT INTO books (id, title, author) VALUES (1, '1984', 'George Orwell');
INSERT INTO books (id, title, author) VALUES (2, 'To Kill a Mockingbird', 'Harper Lee');
INSERT INTO books (id, title, author) VALUES (3, 'The Great Gatsby', 'F. Scott Fitzgerald');

UPDATE books SET author = 'Francis Scott Fitzgerald' WHERE id = 3;

-- Delete the book with id=2
-- Your code here

Hint

Use DELETE FROM books WHERE id = 2; to remove the book.

Practice

1. Which DML command is used to add new rows to a database table?

easy

A. INSERT

B. UPDATE

C. DELETE

D. SELECT

Solution

1. Step 1: Understand the purpose of DML commands

   DML commands are used to manipulate data in tables: adding, changing, or removing rows.

2. Step 2: Identify the command for adding rows

   The INSERT command adds new rows, UPDATE changes existing rows, and DELETE removes rows.

3. Final Answer:

   INSERT -> Option A

4. Quick Check:

   Add new rows = INSERT [OK]

Hint: Add new data? Use INSERT command [OK]

Common Mistakes:

• Confusing UPDATE with INSERT
• Using DELETE to add data
• Thinking SELECT adds data

2. Which of the following is the correct syntax to update the salary to 5000 for employee with id 10?

easy

A. UPDATE employees SET salary 5000 WHERE id = 10;

B. UPDATE employees salary = 5000 WHERE id = 10;

C. UPDATE employees SET salary = 5000 WHERE id = 10;

D. UPDATE employees SET salary = 5000;

Solution

1. Step 1: Recall the UPDATE syntax

   The correct syntax is UPDATE table_name SET column = value WHERE condition; to update specific rows.

2. Step 2: Check each option

   UPDATE employees SET salary = 5000 WHERE id = 10; matches the correct syntax with a WHERE clause to target employee id 10. Options B and C miss the SET keyword or equal sign. UPDATE employees SET salary = 5000; lacks WHERE, so it updates all rows.

3. Final Answer:

   UPDATE employees SET salary = 5000 WHERE id = 10; -> Option C

4. Quick Check:

   UPDATE with SET and WHERE = UPDATE employees SET salary = 5000 WHERE id = 10; [OK]

Hint: UPDATE needs SET and WHERE for specific rows [OK]

Common Mistakes:

• Omitting SET keyword
• Missing WHERE clause
• Wrong assignment syntax

3. Consider the table products with columns id, name, and price. What will be the result of this query?

DELETE FROM products WHERE price < 100;

medium

A. All products with price greater than 100 will be removed.

B. All products with price less than 100 will be removed.

C. No products will be removed because condition is incorrect.

D. Syntax error due to missing semicolon.

Solution

1. Step 1: Understand DELETE with WHERE condition

   The DELETE command removes rows matching the WHERE condition. Here, it targets rows where price is less than 100.

2. Step 2: Analyze the condition and syntax

   The condition price < 100 correctly selects products priced below 100. The semicolon is optional in many systems but good practice.

3. Final Answer:

   All products with price less than 100 will be removed. -> Option B

4. Quick Check:

   DELETE with price < 100 removes matching rows [OK]

Hint: DELETE removes rows matching WHERE condition [OK]

Common Mistakes:

• Confusing less than with greater than
• Thinking DELETE needs all rows
• Assuming syntax error without semicolon

4. Identify the error in this SQL statement:

UPDATE employees SET salary = 6000 WHERE;

medium

A. Missing condition after WHERE clause.

B. Incorrect use of SET keyword.

C. Salary value should be in quotes.

D. UPDATE cannot be used without DELETE.

Solution

1. Step 1: Check the UPDATE statement structure

   UPDATE requires SET to assign new values and WHERE to specify which rows to update.

2. Step 2: Identify the error in the WHERE clause

   The statement ends with WHERE but no condition follows, which is a syntax error.

3. Final Answer:

   Missing condition after WHERE clause. -> Option A

4. Quick Check:

   WHERE must have a condition [OK]

Hint: WHERE must always have a condition [OK]

Common Mistakes:

• Leaving WHERE empty
• Putting value in quotes unnecessarily
• Confusing UPDATE with DELETE

5. You want to increase the price by 10% for all products priced below 50. Which SQL statement correctly does this?

hard

A. UPDATE products SET price = price + 0.10 WHERE price < 50;

B. UPDATE products SET price = price + 10 WHERE price < 50;

C. UPDATE products SET price = price * 0.10 WHERE price < 50;

D. UPDATE products SET price = price * 1.10 WHERE price < 50;

Solution

1. Step 1: Understand the requirement to increase price by 10%

   Increasing by 10% means multiplying the current price by 1.10 (100% + 10%).

2. Step 2: Analyze each option

   UPDATE products SET price = price * 1.10 WHERE price < 50; correctly multiplies price by 1.10 for prices less than 50. UPDATE products SET price = price + 10 WHERE price < 50; adds 10 units, not 10%. UPDATE products SET price = price * 0.10 WHERE price < 50; multiplies by 0.10, reducing price. UPDATE products SET price = price + 0.10 WHERE price < 50; adds 0.10 units, a small increase.

3. Final Answer:

   UPDATE products SET price = price * 1.10 WHERE price < 50; -> Option D

4. Quick Check:

   Multiply price by 1.10 to increase by 10% [OK]

Hint: Multiply by 1.10 to increase by 10% [OK]

Common Mistakes:

• Adding fixed amount instead of percentage
• Multiplying by 0.10 reduces price
• Forgetting WHERE clause