DDL (CREATE, ALTER, DROP) in DBMS Theory - Mini Project: Build & Apply

DDL (CREATE, ALTER, DROP) Basics

📖 Scenario: You are managing a small library database. You need to create a table to store book information, then update the table structure, and finally remove the table when it is no longer needed.

🎯 Goal: Build SQL commands step-by-step to create a table, alter it by adding a new column, and then drop the table.

📋 What You'll Learn

Create a table named Books with columns BookID (integer, primary key) and Title (varchar(100))

Add a new column Author (varchar(100)) to the Books table

Drop the Books table

💡 Why This Matters

🌍 Real World

Database administrators and developers use DDL commands to set up and modify database structures for applications like libraries, stores, or websites.

💼 Career

Knowing how to create, alter, and drop tables is essential for roles involving database management, backend development, and data engineering.

1

Create the Books table

Write a SQL CREATE TABLE statement to create a table called Books with two columns: BookID as an integer primary key, and Title as a varchar of length 100.

DBMS Theory

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

Hint

Use CREATE TABLE Books (BookID INT PRIMARY KEY, Title VARCHAR(100)); to create the table.

2

Add Author column

Write a SQL ALTER TABLE statement to add a new column called Author with type varchar(100) to the existing Books table.

DBMS Theory

CREATE TABLE Books (
    BookID INT PRIMARY KEY,
    Title VARCHAR(100)
);
-- Write the ALTER TABLE statement to add Author column
-- Your code here

Hint

Use ALTER TABLE Books ADD Author VARCHAR(100); to add the new column.

3

Drop the Books table

Write a SQL DROP TABLE statement to remove the Books table from the database.

DBMS Theory

CREATE TABLE Books (
    BookID INT PRIMARY KEY,
    Title VARCHAR(100)
);
ALTER TABLE Books ADD Author VARCHAR(100);
-- Write the DROP TABLE statement for Books
-- Your code here

Hint

Use DROP TABLE Books; to delete the table.

4

Review and finalize DDL commands

Ensure your SQL script includes the CREATE TABLE statement for Books, the ALTER TABLE statement adding the Author column, and the DROP TABLE statement to remove the table.

DBMS Theory

CREATE TABLE Books (
    BookID INT PRIMARY KEY,
    Title VARCHAR(100)
);
ALTER TABLE Books ADD Author VARCHAR(100);
DROP TABLE Books;
-- Review your full DDL script
-- Your code here

Hint

Make sure all three statements are present and correctly written.

Practice

1. What does the CREATE statement do in a database?

easy

A. It makes a new table to store data.

B. It deletes an existing table and its data.

C. It changes the data inside a table.

D. It backs up the database.

Solution

1. Step 1: Understand the purpose of CREATE

   The CREATE statement is used to make new database objects, such as tables, to store data.

2. Step 2: Compare with other statements

   DROP deletes tables, ALTER changes structure, but CREATE makes new tables.

3. Final Answer:

   It makes a new table to store data. -> Option A

4. Quick Check:

   CREATE = Make new table [OK]

Hint: CREATE always makes new tables, not deletes or changes [OK]

Common Mistakes:

• Confusing CREATE with DROP which deletes tables
• Thinking CREATE changes existing data
• Assuming CREATE backs up data

2. Which of the following is the correct syntax to add a new column named age of type INT to an existing table students?

easy

A. ALTER students ADD age INT;

B. ALTER TABLE students ADD COLUMN age INT;

C. DROP TABLE students ADD age INT;

D. CREATE TABLE students ADD age INT;

Solution

1. Step 1: Identify correct ALTER syntax

   The correct syntax to add a column is: ALTER TABLE table_name ADD COLUMN column_name datatype;

2. Step 2: Check each option

   ALTER TABLE students ADD COLUMN age INT; matches the correct syntax. Options B and C misuse CREATE and DROP. ALTER students ADD age INT; misses TABLE keyword.

3. Final Answer:

   ALTER TABLE students ADD COLUMN age INT; -> Option B

4. Quick Check:

   ALTER TABLE + ADD COLUMN = ALTER TABLE students ADD COLUMN age INT; [OK]

Hint: ALTER TABLE ... ADD COLUMN ... is the correct form [OK]

Common Mistakes:

• Omitting TABLE keyword after ALTER
• Using CREATE or DROP instead of ALTER
• Forgetting the COLUMN keyword (optional but clearer)

3. Consider the following SQL commands executed in order:
CREATE TABLE employees (id INT, name VARCHAR(50));
ALTER TABLE employees ADD COLUMN salary INT;
What will be the structure of the employees table after these commands?

medium

A. Table with columns: id, name, salary

B. Table with columns: id, salary

C. Table with columns: id, name

D. Table with columns: name, salary

Solution

1. Step 1: Analyze CREATE TABLE command

   The first command creates a table named employees with columns id and name.

2. Step 2: Analyze ALTER TABLE command

   The second command adds a new column salary to the employees table.

3. Final Answer:

   Table with columns: id, name, salary -> Option A

4. Quick Check:

   CREATE + ALTER ADD COLUMN = id, name, salary [OK]

Hint: ALTER ADD COLUMN adds new columns to existing tables [OK]

Common Mistakes:

• Ignoring the ALTER command effect
• Assuming DROP was used to remove columns
• Thinking ALTER replaces columns instead of adding

4. Identify the error in the following SQL statement:
ALTER TABLE employees DROP salary;

medium

A. Table name is incorrect

B. DROP cannot be used with ALTER TABLE

C. Missing COLUMN keyword after DROP

D. salary column does not exist

Solution

1. Step 1: Understand DROP syntax in ALTER TABLE

   To remove a column, syntax requires: ALTER TABLE table_name DROP COLUMN column_name;

2. Step 2: Check the given statement

   The statement misses the COLUMN keyword after DROP, causing syntax error.

3. Final Answer:

   Missing COLUMN keyword after DROP -> Option C

4. Quick Check:

   ALTER TABLE DROP COLUMN required [OK]

Hint: DROP COLUMN must be used, not just DROP [OK]

Common Mistakes:

• Omitting COLUMN keyword after DROP
• Confusing DROP TABLE with DROP COLUMN
• Assuming DROP alone removes columns

5. You want to permanently delete a table named orders and all its data from the database. Which SQL command should you use?

hard

A. ALTER TABLE orders DELETE;

B. ALTER TABLE orders DROP;

C. CREATE TABLE orders;

D. DROP TABLE orders;

Solution

1. Step 1: Understand the purpose of DROP TABLE

   DROP TABLE deletes the entire table and its data permanently from the database.

2. Step 2: Evaluate the options

   DROP TABLE orders; uses correct syntax. ALTER TABLE orders DROP; and D misuse ALTER. CREATE TABLE orders; creates a table instead of deleting.

3. Final Answer:

   DROP TABLE orders; -> Option D

4. Quick Check:

   DROP TABLE = delete table permanently [OK]

Hint: Use DROP TABLE to delete tables permanently [OK]

Common Mistakes:

• Using ALTER instead of DROP to delete tables
• Confusing CREATE with DROP
• Omitting TABLE keyword in DROP statement