HintsToday

Great question! In SQL, DELETE, TRUNCATE, and DROP are used to remove data—but they work very differently in terms of what they remove, speed, rollback, and usage.

Here’s a quick comparison followed by detailed explanations with examples:

🔍 Quick Comparison

1️⃣ DELETE

• Removes some or all rows from a table
• Can filter rows using WHERE
• Slower because it logs each row for rollback

✅ Example:

-- Delete specific rows
DELETE FROM orders WHERE status = 'cancelled';

-- Delete all rows
DELETE FROM orders;

• You can ROLLBACK if it’s in a transaction.

2️⃣ TRUNCATE

• Removes all rows, like a “reset”
• Cannot use WHERE
• Usually faster than DELETE because it deallocates data pages directly
• Cannot be rolled back in some databases (e.g., MySQL without transactions)

✅ Example:

-- Delete all rows quickly
TRUNCATE TABLE orders;

• Resets auto-increment counters in most databases.

3️⃣ DROP

• Completely removes the table (structure + data + indexes + constraints)
• Cannot be rolled back
• Most destructive

✅ Example:

DROP TABLE orders;

• The orders table is now gone permanently.

🧠 When to Use What?

🔄 Example: Rollback Scenario

BEGIN;

DELETE FROM employees WHERE age > 60;

-- Oops! Rollback.
ROLLBACK;

-- Works!

TRUNCATE TABLE employees;
-- Cannot rollback in MySQL (unless in transactional engine like InnoDB)

Here are interview-style scenario questions based on DELETE, TRUNCATE, DROP, and related SQL concepts. These are commonly asked in data engineering, SQL analyst, and backend developer roles.

🧠 Conceptual and Scenario-Based Questions

🔹 Q1: You want to remove only the rows where status = 'inactive'. Which SQL command would you use?

A. DELETE
B. TRUNCATE
C. DROP
D. ALTER

✅ Answer: A (DELETE) — only DELETE supports a WHERE clause.

🔹 Q2: You need to remove all records from a table quickly, reset the auto-increment value, and ensure minimal logging. What will you use?

A. DELETE
B. TRUNCATE
C. DROP
D. UPDATE

✅ Answer: B (TRUNCATE)

🔹 Q3: What is the difference between DELETE and TRUNCATE in terms of rollback?

🧠 Answer:

• DELETE can be rolled back as it logs each deleted row (transaction-safe).
• TRUNCATE may or may not be rolled back depending on the DBMS (PostgreSQL: ✅; MySQL with InnoDB: ❌ unless within a transaction).

🔹 Q4: You have a table with millions of rows and want to clean it but keep the table structure, constraints, and indexes. What command is most efficient?

🧠 Answer: TRUNCATE TABLE your_table;

🔹 Q5: Which of the following statements about DROP TABLE is true?

A. It deletes all rows but keeps the structure
B. It removes the table and cannot be rolled back
C. It removes only selected rows
D. It resets the auto-increment but keeps data

✅ Answer: B

🔹 Q6: You run this:

BEGIN;
DELETE FROM employees WHERE salary > 100000;
ROLLBACK;

What happens?

🧠 Answer: No rows are removed. The rollback undoes the delete operation.

🔹 Q7: Can you use a WHERE clause with TRUNCATE?

A. Yes
B. No

✅ Answer: B (No)

🔹 Q8: What happens to the FOREIGN KEY relationships when a table is dropped?

🧠 Answer: Dropping a parent table that is referenced by foreign keys will either:

• Fail (with error: “foreign key constraint violation”)
• Cascade drop (if ON DELETE CASCADE is set)

🔹 Q9: You accidentally ran:

TRUNCATE TABLE orders;

Can you undo this?

🧠 Answer: Only if your DB supports transactional DDL (like PostgreSQL) and you ran it inside a transaction block:

BEGIN;
TRUNCATE TABLE orders;
ROLLBACK;

Otherwise, data is lost permanently.

🔹 Q10: You want to reset a test table (users_test) before every test run. Which is best?

A. DELETE
B. DROP + CREATE
C. TRUNCATE
D. ALTER

✅ Answer: C (TRUNCATE)

⚡ Bonus Real-World Question

You are cleaning up a staging table after every batch insert. Which approach ensures the fastest cleanup and also resets auto-increment ID each time?

🧠 Answer: TRUNCATE TABLE staging_data;

Pages: 1 2 3 4