DROP TABLE and ALTER TABLE in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of dropping a table?

Consider a database with a table named Employees. What happens if you run the following SQL command?

DROP TABLE Employees;

What will be the state of the Employees table after this command?

A The table <code>Employees</code> is permanently deleted along with all its data.

B The table <code>Employees</code> is renamed to <code>Employees_backup</code>.

C The table <code>Employees</code> remains but all data inside it is deleted.

D The table <code>Employees</code> is locked and cannot be accessed.

💡 Hint

Think about what DROP means in SQL.

The DROP TABLE command removes the entire table and all its data permanently from the database.

❓ query_result

intermediate

What does this ALTER TABLE command do?

Given a table Products with columns id and name, what is the effect of this command?

ALTER TABLE Products ADD COLUMN price DECIMAL(10,2);

A Adds a new column named <code>price</code> to the <code>Products</code> table to store decimal numbers.

B Deletes the <code>price</code> column from the <code>Products</code> table.

C Changes the data type of the <code>name</code> column to decimal.

D Renames the <code>Products</code> table to <code>price</code>.

💡 Hint

Look at the keywords ADD COLUMN in the command.

The command adds a new column called price with decimal type to the existing Products table.

📝 Syntax

advanced

Which ALTER TABLE command correctly renames a column?

You want to rename the column old_name to new_name in the table Customers. Which of the following commands is correct in standard SQL?

A ALTER TABLE Customers MODIFY COLUMN old_name new_name;

B ALTER TABLE Customers RENAME COLUMN old_name TO new_name;

C ALTER TABLE Customers CHANGE old_name new_name VARCHAR(255);

D ALTER TABLE Customers RENAME old_name new_name;

💡 Hint

Standard SQL uses a specific syntax for renaming columns.

The standard SQL syntax to rename a column is ALTER TABLE table_name RENAME COLUMN old_name TO new_name;. Other options are dialect-specific or invalid.

🔧 Debug

advanced

Why does this DROP TABLE command fail?

Consider the command:

DROP TABLE Orders;

It fails with an error saying the table does not exist. What is the best way to avoid this error?

A Rename the table before dropping it.

B Use <code>DELETE TABLE Orders;</code> instead of DROP.

C Use <code>DROP TABLE IF EXISTS Orders;</code> to drop only if the table exists.

D Restart the database server before running DROP.

💡 Hint

Check if the table exists before dropping it.

The IF EXISTS clause prevents errors if the table does not exist by only dropping it when present.

🧠 Conceptual

expert

What happens when you drop a table with foreign key dependencies?

Table Orders has a foreign key referencing Customers. What happens if you run:

DROP TABLE Customers;

Assuming no special options are used, what is the expected outcome?

A The <code>Customers</code> table is dropped but the <code>Orders</code> table remains with broken references.

B The <code>Customers</code> table is dropped and all related <code>Orders</code> rows are deleted automatically.

C Both tables are dropped automatically.

D The command fails due to foreign key constraints preventing the drop.

💡 Hint

Think about referential integrity rules in relational databases.

Most databases prevent dropping a table if other tables have foreign keys referencing it, to avoid broken references.

Practice

1. What does the SQL command DROP TABLE do?

easy

A. Adds a new column to the table.

B. Changes the data type of a column.

C. Deletes the entire table and all its data permanently.

D. Renames the table without deleting data.

Solution

1. Step 1: Understand the purpose of DROP TABLE

   The DROP TABLE command removes the whole table and all its data from the database permanently.

2. Step 2: Compare with other commands

   ALTER TABLE changes structure but keeps data; DROP TABLE deletes everything.

3. Final Answer:

   Deletes the entire table and all its data permanently. -> Option C

4. Quick Check:

   DROP TABLE = Delete table and data [OK]

Hint: DROP TABLE removes table and data completely [OK]

Common Mistakes:

• Thinking DROP TABLE only deletes data but keeps structure
• Confusing DROP TABLE with ALTER TABLE
• Assuming DROP TABLE can be undone easily

2. Which of the following is the correct syntax to add a new column named age of type INTEGER to a table called users?

easy

A. DROP TABLE users ADD COLUMN age INTEGER;

B. ALTER TABLE users ADD COLUMN age INTEGER;

C. ALTER TABLE users DROP COLUMN age INTEGER;

D. ALTER users ADD age INTEGER;

Solution

1. Step 1: Identify correct ALTER TABLE syntax

   To add a column, use: ALTER TABLE table_name ADD COLUMN column_name data_type;

2. Step 2: Check each option

   ALTER TABLE users ADD COLUMN age INTEGER; matches correct syntax; others misuse DROP or omit TABLE keyword.

3. Final Answer:

   ALTER TABLE users ADD COLUMN age INTEGER; -> Option B

4. Quick Check:

   ALTER TABLE ADD COLUMN = ALTER TABLE users ADD COLUMN age INTEGER; [OK]

Hint: Use ALTER TABLE ... ADD COLUMN to add columns [OK]

Common Mistakes:

• Using DROP TABLE instead of ALTER TABLE
• Omitting TABLE keyword after ALTER
• Using ADD without COLUMN keyword

3. Given the table employees with columns id, name, and salary, what will be the result of this SQL command?

ALTER TABLE employees DROP COLUMN salary;

medium

A. The table employees will be deleted.

B. All data in the employees table will be deleted.

C. A new column named salary will be added.

D. The salary column will be removed from the employees table.

Solution

1. Step 1: Understand ALTER TABLE DROP COLUMN

   This command removes the specified column from the table structure but keeps the table and other data intact.

2. Step 2: Analyze options

   Only The salary column will be removed from the employees table. correctly describes removing the salary column; others describe deleting data or table, or adding columns.

3. Final Answer:

   The salary column will be removed from the employees table. -> Option D

4. Quick Check:

   ALTER TABLE DROP COLUMN removes column [OK]

Hint: ALTER TABLE DROP COLUMN removes only that column [OK]

Common Mistakes:

• Thinking DROP COLUMN deletes all data
• Confusing DROP COLUMN with DROP TABLE
• Assuming DROP COLUMN adds a column

4. What is wrong with this SQL command?

ALTER TABLE customers DROP age;

medium

A. The command is missing the keyword COLUMN after DROP.

B. DROP cannot be used with ALTER TABLE.

C. The table name is incorrect.

D. The command should be DROP TABLE customers;

Solution

1. Step 1: Check ALTER TABLE DROP syntax

   To remove a column, the syntax requires DROP COLUMN column_name, not just DROP column_name.

2. Step 2: Validate other options

   DROP is valid with ALTER TABLE if used correctly; table name is correct; DROP TABLE deletes whole table, not column.

3. Final Answer:

   The command is missing the keyword COLUMN after DROP. -> Option A

4. Quick Check:

   ALTER TABLE DROP COLUMN required [OK]

Hint: Use DROP COLUMN, not just DROP, to remove columns [OK]

Common Mistakes:

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

5. You want to rename the column fullname to name in the table students. Which SQL command correctly does this in a standard way?

hard

A. ALTER TABLE students RENAME COLUMN fullname TO name;

B. ALTER TABLE students CHANGE fullname name VARCHAR(100);

C. DROP TABLE students RENAME COLUMN fullname TO name;

D. ALTER TABLE students DROP COLUMN fullname ADD COLUMN name VARCHAR(100);

Solution

1. Step 1: Identify standard rename column syntax

   The standard SQL command to rename a column is ALTER TABLE table_name RENAME COLUMN old_name TO new_name;

2. Step 2: Evaluate options

   ALTER TABLE students RENAME COLUMN fullname TO name; matches standard syntax; A wrongly uses DROP TABLE; B is MySQL-specific CHANGE syntax; D drops and adds column losing data.

3. Final Answer:

   ALTER TABLE students RENAME COLUMN fullname TO name; -> Option A

4. Quick Check:

   Rename column = ALTER TABLE RENAME COLUMN [OK]

Hint: Use ALTER TABLE RENAME COLUMN to rename columns safely [OK]

Common Mistakes:

• Using DROP TABLE instead of ALTER TABLE
• Dropping and adding columns loses data
• Using non-standard syntax without checking compatibility