DROP and TRUNCATE behavior in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the DROP command do in MySQL?

DROP removes a table and all its data permanently from the database. The table structure and data are deleted.

beginner

What is the main difference between DROP and TRUNCATE?

DROP deletes the entire table and its data, while TRUNCATE deletes only the data but keeps the table structure for reuse.

intermediate

Does TRUNCATE in MySQL log individual row deletions?

No, TRUNCATE is faster because it does not log individual row deletions. It deallocates data pages instead.

intermediate

Can you rollback a DROP command in MySQL?

No, DROP is a DDL command and cannot be rolled back once executed.

intermediate

What happens to auto-increment counters when you TRUNCATE a table?

TRUNCATE resets the auto-increment counter to its start value in MySQL.

Which command removes a table and its data permanently in MySQL?

A UPDATE

B TRUNCATE

C DELETE

D DROP

DROP deletes the entire table and its data permanently.

What does TRUNCATE do to the table structure?

A Renames the table

B Deletes the table structure

C Keeps the table structure intact

D Backs up the table

TRUNCATE deletes data but keeps the table structure for reuse.

Can you undo a TRUNCATE command using ROLLBACK in MySQL?

A No, TRUNCATE cannot be rolled back

B Only for InnoDB tables

C Only if inside a transaction

D Yes, always

TRUNCATE is a DDL command and cannot be rolled back.

Which command is faster for deleting all rows in a table?

A DELETE without WHERE

B TRUNCATE

C DROP

D UPDATE

TRUNCATE is faster because it deallocates data pages without logging each row deletion.

What happens to the auto-increment value after TRUNCATE?

A It resets to the start value

B It doubles

C It stays the same

D It becomes NULL

TRUNCATE resets the auto-increment counter to its initial value.

Explain the differences between DROP and TRUNCATE commands in MySQL.

Think about what happens to the table and data after each command.

Describe what happens internally when you use TRUNCATE on a MySQL table.

Focus on performance and logging differences.

Practice

1. What is the main difference between DROP and TRUNCATE commands in MySQL?

easy

A. DROP only deletes data, TRUNCATE deletes the table structure.

B. TRUNCATE deletes the entire table and its data, DROP deletes only the data but keeps the table.

C. DROP deletes the entire table and its data, TRUNCATE deletes only the data but keeps the table.

D. DROP and TRUNCATE do exactly the same thing.

Solution

1. Step 1: Understand DROP behavior

   DROP removes the entire table including its structure and data permanently.

2. Step 2: Understand TRUNCATE behavior

   TRUNCATE removes all rows but keeps the table structure for future use.

3. Final Answer:

   DROP deletes the entire table and its data, TRUNCATE deletes only the data but keeps the table. -> Option C

4. Quick Check:

   DROP removes table, TRUNCATE empties table [OK]

Hint: DROP removes table, TRUNCATE empties data only [OK]

Common Mistakes:

• Confusing which command deletes the table structure
• Thinking TRUNCATE deletes the table itself
• Assuming DROP only deletes data

2. Which of the following is the correct syntax to remove all rows from a table named employees without deleting the table itself?

easy

A. DROP TABLE employees;

B. TRUNCATE TABLE employees;

C. DELETE TABLE employees;

D. REMOVE TABLE employees;

Solution

1. Step 1: Identify command to remove all rows but keep table

   TRUNCATE TABLE removes all rows quickly without deleting the table.

2. Step 2: Check syntax correctness

   TRUNCATE TABLE employees; is the correct syntax. DROP TABLE deletes the table, DELETE TABLE and REMOVE TABLE are invalid.

3. Final Answer:

   TRUNCATE TABLE employees; -> Option B

4. Quick Check:

   TRUNCATE syntax correct for emptying table [OK]

Hint: Use TRUNCATE TABLE to empty without deleting [OK]

Common Mistakes:

• Using DROP instead of TRUNCATE to empty table
• Using DELETE TABLE which is invalid syntax
• Confusing REMOVE TABLE as a valid command

3. Given a table products with 1000 rows, what will be the result after running TRUNCATE TABLE products;?

medium

A. All rows are removed but the table structure remains.

B. The table products is deleted completely.

C. Only half of the rows are deleted.

D. Syntax error occurs.

Solution

1. Step 1: Understand effect of TRUNCATE on data

   TRUNCATE TABLE removes all rows from the table quickly.

2. Step 2: Understand effect on table structure

   The table structure remains intact and ready for new data.

3. Final Answer:

   All rows are removed but the table structure remains. -> Option A

4. Quick Check:

   TRUNCATE empties table, keeps structure [OK]

Hint: TRUNCATE clears data, keeps table ready [OK]

Common Mistakes:

• Thinking TRUNCATE deletes the table itself
• Assuming partial row deletion
• Expecting syntax error

4. You want to remove a table named orders completely, but accidentally run TRUNCATE orders;. What is the problem with this command?

medium

A. It will cause a syntax error because TABLE keyword is missing.

B. It will delete the table and all its data.

C. It will only delete some rows, not all.

D. It will rename the table instead of deleting.

Solution

1. Step 1: Check syntax of TRUNCATE command

   The correct syntax requires the keyword TABLE: TRUNCATE TABLE orders;.

2. Step 2: Identify error from missing keyword

   Omitting TABLE causes a syntax error in MySQL.

3. Final Answer:

   It will cause a syntax error because TABLE keyword is missing. -> Option A

4. Quick Check:

   TRUNCATE requires TABLE keyword [OK]

Hint: Always include TABLE keyword in TRUNCATE [OK]

Common Mistakes:

• Omitting TABLE keyword in TRUNCATE
• Expecting TRUNCATE to delete table structure
• Confusing TRUNCATE with DROP syntax

5. You have a table customers with foreign key constraints referencing it from other tables. Which command should you use to quickly remove all data from customers without dropping the table and without breaking foreign key constraints?

hard

A. ALTER TABLE customers DROP;

B. TRUNCATE TABLE customers;

C. DROP TABLE customers;

D. DELETE FROM customers;

Solution

1. Step 1: Understand foreign key constraints effect

   TRUNCATE TABLE cannot be used if foreign keys reference the table; it will cause an error.

2. Step 2: Choose command that removes data safely

   DELETE FROM customers; removes all rows and respects foreign key constraints.

3. Step 3: Confirm other options

   DROP TABLE deletes the table, breaking references; ALTER TABLE DROP is invalid syntax.

4. Final Answer:

   DELETE FROM customers; -> Option D

5. Quick Check:

   DELETE respects foreign keys, TRUNCATE does not [OK]

Hint: Use DELETE when foreign keys exist, not TRUNCATE [OK]

Common Mistakes:

• Using TRUNCATE despite foreign key constraints
• Using DROP which deletes the table
• Trying invalid ALTER TABLE DROP syntax