SQL Interview Questions !!

🎗 Write a query to find all employees whose salaries exceed the company's average salary.
🎗 Write a query to retrieve the names of employees who work in the same department as 'John Doe'.
🎗 Write a query to display the second highest salary from the Employee table without using the MAX function twice.
🎗 Write a query to find all customers who have placed more than five orders.
🎗 Write a query to count the total number of orders placed by each customer.
🎗 Write a query to list employees who joined the company within the last 6 months.
🎗 Write a query to calculate the total sales amount for each product.
🎗 Write a query to list all products that have never been sold.
🎗 Write a query to remove duplicate rows from a table.
🎗 Write a query to identify the top 10 customers who have not placed any orders in the past year.

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🗄️ SQL Developer Roadmap

📂 SQL Basics (SELECT, WHERE, ORDER BY)
∟📂 Joins (INNER, LEFT, RIGHT, FULL)
∟📂 Aggregate Functions (COUNT, SUM, AVG)
∟📂 Grouping Data (GROUP BY, HAVING)
∟📂 Subqueries & Nested Queries
∟📂 Data Modification (INSERT, UPDATE, DELETE)
∟📂 Database Design (Normalization, Keys)
∟📂 Indexing & Query Optimization
∟📂 Stored Procedures & Functions
∟📂 Transactions & Locks
∟📂 Views & Triggers
∟📂 Backup & Restore
∟📂 Working with NoSQL basics (optional)
∟📂 Real Projects & Practice
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SQL interview questions with answers 😄👇

1. Question: What is SQL?

Answer: SQL (Structured Query Language) is a programming language designed for managing and manipulating relational databases. It is used to query, insert, update, and delete data in databases.

2. Question: Differentiate between SQL and MySQL.

Answer: SQL is a language for managing relational databases, while MySQL is an open-source relational database management system (RDBMS) that uses SQL as its language.

3. Question: Explain the difference between INNER JOIN and LEFT JOIN.

Answer: INNER JOIN returns rows when there is a match in both tables, while LEFT JOIN returns all rows from the left table and the matched rows from the right table, filling in with NULLs for non-matching rows.

4. Question: How do you remove duplicate records from a table?

Answer: Use the DISTINCT keyword in a SELECT statement to retrieve unique records. For example: SELECT DISTINCT column1, column2 FROM table;

5. Question: What is a subquery in SQL?

Answer: A subquery is a query nested inside another query. It can be used to retrieve data that will be used in the main query as a condition to further restrict the data to be retrieved.

6. Question: Explain the purpose of the GROUP BY clause.

Answer: The GROUP BY clause is used to group rows that have the same values in specified columns into summary rows, like when using aggregate functions such as COUNT, SUM, AVG, etc.

7. Question: How can you add a new record to a table?

Answer: Use the INSERT INTO statement. For example: INSERT INTO table_name (column1, column2) VALUES (value1, value2);

8. Question: What is the purpose of the HAVING clause?

Answer: The HAVING clause is used in combination with the GROUP BY clause to filter the results of aggregate functions based on a specified condition.

9. Question: Explain the concept of normalization in databases.

Answer: Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves breaking down tables into smaller, related tables.

10. Question: How do you update data in a table in SQL?

Answer: Use the UPDATE statement to modify existing records in a table. For example: UPDATE table_name SET column1 = value1 WHERE condition;

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✅ Useful SQL Concepts You Should Know 🚀📈

1️⃣ Constraints in SQL:

- PRIMARY KEY – Uniquely identifies each row
- FOREIGN KEY – Links to another table
- UNIQUE – Ensures all values are different
- NOT NULL – Column must have a value
- CHECK – Validates data before insert/update

2️⃣ SQL Views:

Virtual tables based on result of a query
CREATE VIEW top_students AS
SELECT name, marks FROM students WHERE marks > 90;
3️⃣ Indexing:

Improves query performance
CREATE INDEX idx_name ON employees(name);
4️⃣ SQL Transactions:

Ensure data integrity
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;
5️⃣ Triggers:

Automatic actions when events occur
CREATE TRIGGER log_update
AFTER UPDATE ON employees
FOR EACH ROW
INSERT INTO logs(action) VALUES ('Employee updated');
6️⃣ Stored Procedures:

Reusable blocks of SQL logic
CREATE PROCEDURE getTopStudents()
BEGIN
SELECT FROM students WHERE marks > 90;
END;
7️⃣ Common Table Expressions (CTEs):

Temporary named result sets
WITH dept_count AS (
SELECT department, COUNT() AS total FROM employees GROUP BY department
)
SELECT FROM dept_count;
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SQL Interview Questions with Answers Part-1: ☑️

1. What is SQL? 
   SQL (Structured Query Language) is a standardized programming language designed to manage and manipulate relational databases. It allows you to query, insert, update, and delete data, as well as create and modify schema objects like tables and views.

2. Differentiate between SQL and NoSQL databases. 
   SQL databases are relational, table-based, and use structured query language with fixed schemas, ideal for complex queries and transactions. NoSQL databases are non-relational, can be document, key-value, graph, or column-oriented, and are schema-flexible, designed for scalability and handling unstructured data.

3. What are the different types of SQL commands?
⦁ DDL (Data Definition Language): CREATE, ALTER, DROP (define and modify structure)
⦁ DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE (data operations)
⦁ DCL (Data Control Language): GRANT, REVOKE (permission control)
⦁ TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT (transaction management)

4. Explain the difference between WHERE and HAVING clauses.
⦁ WHERE filters rows before grouping (used with SELECT, UPDATE).
⦁ HAVING filters groups after aggregation (used with GROUP BY), e.g., filtering aggregated results like sums or counts.

5. Write a SQL query to find the second highest salary in a table. 
   Using a subquery:

SELECT MAX(salary) FROM employees  
WHERE salary < (SELECT MAX(salary) FROM employees);

Or using DENSE_RANK():

SELECT salary FROM (  
  SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) as rnk  
  FROM employees) t  
WHERE rnk = 2;

6. What is a JOIN? Explain different types of JOINs. 
   A JOIN combines rows from two or more tables based on a related column:
⦁ INNER JOIN: returns matching rows from both tables.
⦁ LEFT JOIN (LEFT OUTER JOIN): all rows from the left table, matched rows from right.
⦁ RIGHT JOIN (RIGHT OUTER JOIN): all rows from right table, matched rows from left.
⦁ FULL JOIN (FULL OUTER JOIN): all rows when there’s a match in either table.
⦁ CROSS JOIN: Cartesian product of both tables.

7. How do you optimize slow-performing SQL queries?
⦁ Use indexes appropriately to speed up lookups.
⦁ Avoid SELECT *; only select necessary columns.
⦁ Use joins carefully; filter early with WHERE clauses.
⦁ Analyze execution plans to identify bottlenecks.
⦁ Avoid unnecessary subqueries; use EXISTS or JOINs.
⦁ Limit result sets with pagination if dealing with large datasets.

8. What is a primary key? What is a foreign key?
⦁ Primary Key: A unique identifier for records in a table; it cannot be NULL.
⦁ Foreign Key: A field that creates a link between two tables by referring to the primary key in another table, enforcing referential integrity.

9. What are indexes? Explain clustered and non-clustered indexes.
⦁ Indexes speed up data retrieval by providing quick lookups.
⦁ Clustered Index: Sorts and stores the actual data rows in the table based on the key; a table can have only one clustered index.
⦁ Non-Clustered Index: Creates a separate structure that points to the data rows; tables can have multiple non-clustered indexes.

10. Write a SQL query to fetch the top 5 records from a table. 
    In SQL Server and PostgreSQL:

SELECT * FROM table_name  
ORDER BY some_column DESC  
LIMIT 5;  

In SQL Server (older syntax):

SELECT TOP 5 * FROM table_name  
ORDER BY some_column DESC;  

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✅ Stored Procedures 💯

🧠 1. What is a Stored Procedure?
A Stored Procedure is a saved SQL program
• Stored inside the database
• Can be executed anytime

Think like this 👇
“Reusable SQL code block”

⚡ 2. Why Use Stored Procedures?
• Reuse SQL logic
• Reduce repeated code
• Better security
• Faster execution for repeated tasks

⚡ 3. Basic Syntax

👉 MySQL Example:
DELIMITER //

CREATE PROCEDURE GetEmployees()
BEGIN
SELECT * FROM employees;
END //

DELIMITER ;

▶️ 4. Execute Stored Procedure
CALL GetEmployees();

🔥 5. Procedure with Parameter
DELIMITER //

CREATE PROCEDURE GetDeptEmployees(IN dept_name VARCHAR(50))
BEGIN
SELECT * FROM employees
WHERE department = dept_name;
END //

DELIMITER ;

▶️ 6. Execute Parameterized Procedure
CALL GetDeptEmployees('IT');

❌ 7. Drop Stored Procedure
DROP PROCEDURE GetEmployees;

🎯 8. Real Example
Increase salary for all IT employees:

DELIMITER //

CREATE PROCEDURE IncreaseSalary()
BEGIN
UPDATE employees
SET salary = salary + 5000
WHERE department = 'IT';
END //

DELIMITER ;

🎯 9. Practice Tasks
1. Create procedure to show all employees
2. Create procedure for HR employees
3. Create procedure with salary parameter
4. Execute stored procedure
5. Drop procedure

⚡ Mini Challenge 🔥

Create procedure to return employees with salary > given value

✅ Solution

DELIMITER //

CREATE PROCEDURE GetHighSalaryEmployees(IN min_salary INT)
BEGIN
SELECT *
FROM employees
WHERE salary > min_salary;
END //

DELIMITER ;

▶️ Execute Procedure

CALL GetHighSalaryEmployees(50000);

✔ Returns employees earning more than 50k

Key Difference:
• Function → returns value
• Procedure → performs action / multiple operations

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🔥 Now, let’s move to the next topic:

Triggers in SQL
(Automation inside database 💯)

🧠 1. What is a Trigger?
A Trigger is a special SQL block
👉 that runs automatically
👉 when an event happens in a table

Think like this 👇
👉 “Automatic action on INSERT / UPDATE / DELETE”

⚡ 2. Why Use Triggers?
✔ Automatic logging
✔ Data validation
✔ Audit tracking
✔ Prevent invalid operations

⚡ 3. Types of Triggers
BEFORE INSERT → Runs before inserting data
AFTER INSERT → Runs after inserting data
BEFORE UPDATE → Runs before updating
AFTER UPDATE → Runs after updating
BEFORE DELETE → Runs before deleting
AFTER DELETE → Runs after deleting

🔥 4. Basic Trigger Example
👉 Automatically log inserted employee

CREATE TABLE employee_log (
log_message VARCHAR(255)
);

DELIMITER //

CREATE TRIGGER after_employee_insert
AFTER INSERT ON employees
FOR EACH ROW
BEGIN
INSERT INTO employee_log
VALUES (CONCAT('New employee added: ', NEW.name));
END //

DELIMITER ;

🧠 5. Important Keywords
NEW → New inserted/updated value
OLD → Previous value before update/delete

⚡ 6. BEFORE UPDATE Example
👉 Prevent negative salary
DELIMITER //

CREATE TRIGGER check_salary
BEFORE UPDATE ON employees
FOR EACH ROW
BEGIN
IF NEW.salary < 0 THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Salary cannot be negative';
END IF;
END //

DELIMITER ;

❌ 7. Drop Trigger
DROP TRIGGER after_employee_insert;

🎯 8. Practice Tasks
1. Create AFTER INSERT trigger
2. Create BEFORE UPDATE trigger
3. Prevent negative salary using trigger
4. Log deleted employees
5. Drop created trigger

⚡ Mini Challenge 🔥
👉 Create trigger to automatically save deleted employee names into another table

🔥 Mini Challenge Solution

👉 Automatically save deleted employee names into another table

✅ Step 1: Create Log Table

CREATE TABLE deleted_employees (
emp_id INT,
name VARCHAR(50),
deleted_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

✅ Step 2: Create Trigger

DELIMITER //

CREATE TRIGGER log_deleted_employee
AFTER DELETE ON employees
FOR EACH ROW
BEGIN
INSERT INTO deleted_employees(emp_id, name)
VALUES (OLD.emp_id, OLD.name);
END //

DELIMITER ;

🧠 How It Works

👉 AFTER DELETE → runs automatically after deletion

👉 OLD.emp_id and OLD.name
Access deleted row values before they disappear

✅ Example

DELETE FROM employees
WHERE emp_id = 101;

✔ Deleted employee info automatically saved in deleted_employees table 💯

🔥 Pro Tip
Triggers are powerful but:
❌ Too many triggers can slow database
✅ Use them carefully 💯

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