DROP TABLE and ALTER TABLE in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL command DROP TABLE do?

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

beginner

How do you add a new column to an existing table in SQL?

Use ALTER TABLE table_name ADD column_name datatype; to add a new column to a table.

intermediate

What happens if you try to DROP TABLE a table that does not exist?

You get an error unless you use DROP TABLE IF EXISTS table_name; which avoids the error if the table is missing.

intermediate

How can you rename a table using SQL?

Use ALTER TABLE old_table_name RENAME TO new_table_name; to rename a table.

intermediate

Can you remove a column from a table using ALTER TABLE? How?

Yes, use ALTER TABLE table_name DROP COLUMN column_name; to remove a column from a table.

What does DROP TABLE employees; do?

A Deletes the employees table and all its data

B Deletes only the data inside employees table

C Renames the employees table

D Adds a new column to employees table

DROP TABLE removes the entire table and its data permanently.

Which SQL command adds a new column named age to a table users?

A DROP TABLE users ADD age;

B ALTER TABLE users DROP COLUMN age;

C ALTER TABLE users ADD age INT;

D CREATE COLUMN age ON users;

ALTER TABLE users ADD age INT; adds a new column called age of type integer.

How do you rename a table old_name to new_name?

A ALTER TABLE old_name RENAME TO new_name;

B DROP TABLE old_name RENAME TO new_name;

C RENAME TABLE old_name TO new_name;

D ALTER TABLE old_name CHANGE TO new_name;

The correct syntax to rename a table is ALTER TABLE old_name RENAME TO new_name;.

What happens if you run DROP TABLE IF EXISTS my_table; and my_table does not exist?

A Table is renamed

B Error occurs because table is missing

C Table is created

D No error, command runs safely

IF EXISTS prevents errors if the table does not exist.

Which command removes a column named address from the customers table?

A DROP TABLE customers.address;

B ALTER TABLE customers DROP COLUMN address;

C ALTER TABLE customers REMOVE address;

D DELETE COLUMN address FROM customers;

Use ALTER TABLE customers DROP COLUMN address; to remove a column.

Explain what the DROP TABLE command does and when you might use it.

Think about removing a whole spreadsheet from your files.

Describe how to use ALTER TABLE to change a table structure, including adding and removing columns.

Imagine changing the columns in a spreadsheet without losing the data.

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