🧠 Common Beginner Mistakes

❌ Mistake 1: Forgetting FROM

Wrong: SELECT customer_name;

Correct: SELECT customer_name FROM customers;

❌ Mistake 2: Using commas incorrectly

Wrong: SELECT customer_id customer_name city

Correct: Use commas

❌ Mistake 3: Using quotes around column names unnecessarily

SELECT 'customer_name' treats it as text, not column.

❌ Mistake 4: Confusing *

SELECT * means return all columns, not all rows.

🎯 Practice Questions

Q1. Display all columns from employees

Q2. Display employee_name, salary, department_id

Q3. Display unique cities from customers

Q4. Display product_name, price and 15% discounted price

Q5. Display product_name, selling_price, cost_price and profit

Q6. Display customer names with alias Customer

Q7. Display employee name and salary increased by 10%

✅ Answers

-- A1
SELECT * FROM employees;

-- A2
SELECT employee_name, salary, department_id FROM employees;

-- A3
SELECT DISTINCT city FROM customers;

-- A4
SELECT product_name, price, price * 0.85 AS discounted_price FROM products;

-- A5
SELECT product_name, selling_price, cost_price, selling_price - cost_price AS profit FROM products;

-- A6
SELECT customer_name AS Customer FROM customers;

-- A7
SELECT employee_name, salary, salary * 1.10 AS increased_salary FROM employees;

💼 Interview Questions

1. What does SELECT do? → Retrieves data from columns.

2. What does SELECT * mean? → Retrieves all columns.

3. What is DISTINCT? → Removes duplicate combinations.

4. What is an alias? → Temporary name for clarity.

5. Can SQL perform calculations? → Yes, arithmetic expressions directly in queries.

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