🧠 SQL Projects & TestDome Certification

📜 Overview

This repository showcases my SQL learning journey, hands-on problem solving, and the official TestDome SQL Certificate that places me in the Top 25% globally (March 14, 2025).

I developed and tested all SQL scripts here as part of practical exercises designed to simulate real-world database tasks, including data creation, manipulation, joins, aggregation, and relational modeling.

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🏆 Certification

Platform: TestDome
Exam: SQL Skills Assessment
Result: Top 25% (March 2025)
Name: Erietta Chamalidou

The TestDome assessment evaluates practical SQL proficiency through real coding scenarios that measure problem-solving ability and efficiency in working with data.

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📚 What I Learned

Through the exercises and certification process, I strengthened my ability to:

• Design normalized relational databases (CREATE TABLE, PRIMARY KEY, FOREIGN KEY)
• Perform complex joins and set operations (INNER JOIN, LEFT JOIN, UNION)
• Implement data manipulation queries (INSERT, UPDATE, DELETE)
• Use aggregate functions (COUNT, AVG, GROUP BY, HAVING)
• Apply conditional logic and handle case sensitivity in SQL
• Model self-referential relationships (e.g., employee–manager hierarchy)
• Write clean, reusable, and efficient SQL code

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💻 Sample Highlights

1️⃣ Student Records and Case Sensitivity

SELECT COUNT(*) 
FROM students 
WHERE LOWER(first_name) = 'dimitra';

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2️⃣ Updating Enrollments by Range

UPDATE enrollments
SET year = 2015
WHERE enrollment_id BETWEEN 20 AND 100;

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3️⃣ Merging Distinct Animal Names

SELECT dogsname FROM dogs
UNION
SELECT catsname FROM cats;

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4️⃣ Friendships and Self-Referential Relationships

CREATE TABLE friends (
  user1 INT REFERENCES users(id),
  user2 INT REFERENCES users(id),
  PRIMARY KEY (user1, user2),
  CHECK (user1 < user2)
);

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5️⃣ Calculating Average Session Durations

SELECT user_id, AVG(duration)
FROM sessions
GROUP BY user_id
HAVING COUNT(user_id) > 1;

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6️⃣ Seller–Item Join

SELECT sellers.name, items.name
FROM sellers
INNER JOIN items ON sellers.id = items.sellerId
WHERE sellers.rating > 4;

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7️⃣ Finding Non-Manager Employees

SELECT name 
FROM employees
WHERE id NOT IN (
  SELECT manager_id FROM employees WHERE manager_id IS NOT NULL
);

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🧩 Tools & Technologies

• PostgreSQL & MySQL
• VS Code SQL Extension

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🌟 Achievements

• 🥇 Ranked Top 25% worldwide in TestDome SQL
• 🧩 Built structured SQL solutions reflecting real industry cases

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🙋‍♀️ About Me

I’m Erietta Chamalidou, a COBOL Developer and SQL enthusiast with a passion for database systems, clean code, and continuous learning.
You can connect with me to collaborate on projects related to:

• Database design and optimization
• Backend testing
• Financial systems and data analytics