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🧠 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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