Ace Your SQL Interview in 2025!

SQL remains a crucial skill for developers, data analysts, and database administrators. Here are some must-know SQL interview questions to help you prepare:

🔹 Basic SQL Questions

✅ What are the different types of SQL commands (DDL, DML, DCL, TCL)?
✅ What is the difference between DELETE, TRUNCATE, and DROP?

✅ What are Primary Key and Foreign Key?

✅ Explain different types of joins in SQL.

✅ What is Normalization, and why is it important?

🔹 Intermediate SQL Questions

✅ What is the difference between HAVING and WHERE?

✅ How does GROUP BY work in SQL?

✅ Explain ACID properties in a database.

✅ What is an Index, and how does it improve performance?
✅ What is a stored procedure, and how is it different from a function?

🔹 Advanced SQL Questions

✅ How does a database handle deadlocks, and how can they be prevented?
✅ What are Common Table Expressions (CTEs) and when should you use them?
✅ What is sharding, and how does it help in database scaling?

✅ How does window functions work in SQL?

✅ How do you optimize a slow SQL query?

SQL interviews in 2025 focus on performance optimization, real-world database design, and handling large datasets efficiently.

Practice Verified Codes and Commands

1. Basic SQL Commands:

-- Create a table
CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
FirstName VARCHAR(50),
LastName VARCHAR(50),
DepartmentID INT
);

-- Insert data
INSERT INTO Employees (EmployeeID, FirstName, LastName, DepartmentID)
VALUES (1, 'John', 'Doe', 101);

-- Select data
SELECT * FROM Employees;

-- Update data
UPDATE Employees
SET LastName = 'Smith'
WHERE EmployeeID = 1;

-- Delete data
DELETE FROM Employees
WHERE EmployeeID = 1;

2. Intermediate SQL Commands:

-- GROUP BY example
SELECT DepartmentID, COUNT(*) AS EmployeeCount
FROM Employees
GROUP BY DepartmentID;

-- HAVING example
SELECT DepartmentID, COUNT(<em>) AS EmployeeCount
FROM Employees
GROUP BY DepartmentID
HAVING COUNT(</em>) > 5;

-- Index example
CREATE INDEX idx_lastname
ON Employees (LastName);

3. Advanced SQL Commands:

-- CTE example
WITH DepartmentEmployeeCount AS (
SELECT DepartmentID, COUNT(*) AS EmployeeCount
FROM Employees
GROUP BY DepartmentID
)
SELECT * FROM DepartmentEmployeeCount;

-- Window function example
SELECT EmployeeID, FirstName, LastName, DepartmentID,
ROW_NUMBER() OVER (PARTITION BY DepartmentID ORDER BY EmployeeID) AS RowNum
FROM Employees;

-- Query optimization example
EXPLAIN PLAN FOR
SELECT * FROM Employees WHERE DepartmentID = 101;

What Undercode Say

SQL is a powerful language for managing and manipulating relational databases. Mastering SQL is essential for anyone working with data, whether you’re a developer, data analyst, or database administrator. The key to excelling in SQL interviews lies in understanding both the theoretical concepts and practical applications.

1. Basic Commands: Start with the basics like creating tables, inserting data, and simple queries. These form the foundation of SQL.
2. Intermediate Concepts: Learn about grouping data, using indexes, and understanding the ACID properties of transactions. These concepts are crucial for handling more complex data operations.
3. Advanced Techniques: Dive into advanced topics like CTEs, window functions, and query optimization. These are often the focus of technical interviews and are essential for working with large datasets.

Linux Commands for Database Management:

• PostgreSQL: `psql -U username -d dbname` to connect to a PostgreSQL database.
• MySQL: `mysql -u username -p` to connect to a MySQL database.
• Backup: `pg_dump dbname > backup.sql` for PostgreSQL, `mysqldump -u username -p dbname > backup.sql` for MySQL.

Windows Commands for Database Management:

• Start MySQL Service: `net start mysql`
  – Stop MySQL Service: `net stop mysql`
  – Check PostgreSQL Service: `sc query postgresql-x64-13`

Useful URLs:

• SQL Tutorial
• PostgreSQL Documentation
• MySQL Documentation

By combining theoretical knowledge with practical skills, you can confidently tackle any SQL interview question and excel in your career. Keep practicing, and don’t hesitate to explore more advanced topics as you progress.

References:

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