SQL Practice Repository

My personal collection of SQL exercises for continuous learning and interview prep. Built this to practice everything from basic queries to advanced analytics through realistic business scenarios.

What's Inside

7 comprehensive modules covering beginner through advanced SQL:

Core Modules (Beginner → Intermediate)

1. Basic SELECT - Employee management system
   WHERE, ORDER BY, LIKE patterns, filtering
2. JOINs - Company database with departments/employees
   INNER/LEFT/RIGHT JOINs, self-joins, multiple tables
3. GROUP BY & Aggregates - E-commerce sales analysis
   COUNT/SUM/AVG, HAVING, business metrics, percentages
4. Subqueries - University enrollment system
   WHERE/SELECT subqueries, EXISTS, correlated queries
5. NULL Handling - Customer support system
   IS NULL, COALESCE, NULLIF, edge cases

Advanced Modules

6. Window Functions - Sales analytics & BI
   Rankings, LAG/LEAD, moving averages, PARTITION BY
7. CTEs - Multi-level business reporting
   Basic & recursive CTEs, hierarchical data, query organization

Learning Paths

learning/ folder - Learn by example

• Complete worked examples with explanations
• Schema + data + solutions all in one place
• Perfect for understanding patterns and building fundamentals

interview-prep/ folder - Test yourself

• Challenge files with problems only (no solutions visible)
• Separate solution files to check your work
• Realistic interview simulation

Quick Start

Option 1: Simple SQLite setup (recommended)

# Set up database with all practice data
sqlite3 practice.db < setup_database.sql

# Start practicing immediately
sqlite3 practice.db
sqlite> SELECT * FROM employees LIMIT 3;

Option 2: Online tools

• DB Fiddle, SQLite Online, or W3Schools SQL Tryit
• Copy/paste the CREATE and INSERT statements from any file

My Learning Approach

For building fundamentals:

1. Start with learning/01_basic_select.sql - get comfortable with syntax
2. Progress through modules 2-5 at your own pace
3. Each file has realistic scenarios, not boring textbook examples

For interview prep:

1. Jump to interview-prep/ challenges when ready
2. Time yourself - aim for 15-30 minutes per complex query
3. Only check solutions after attempting problems yourself

For advanced skills:

1. Master modules 6-7 (Window Functions & CTEs)
2. These are crucial for modern data roles
3. Practice building business dashboards and analytics

Learning Enhancements

Built some extra tools to level up the learning experience:

enhancements/ - Learning Support Tools

Self-Assessment Quiz (self_assessment_quiz.md)

• Multiple-choice questions for all 7 modules
• Progressive difficulty with detailed explanations
• Track your scores and identify weak spots

Query Explanations Guide (query_explanations.md)

• Step-by-step breakdowns of complex queries
• Deep dives into window functions and CTEs
• Understand the "why" behind advanced SQL patterns

Common Mistakes Guide (common_mistakes.md)

• Learn from typical SQL errors and pitfalls
• Wrong vs. right approaches with explanations
• Quick reference for debugging and interview prep

Progress Tracker (progress_tracker.md)

• Gamified learning with achievements and milestones
• Track completion and set personal goals
• Interview readiness assessment

How I Use the Enhancements

1. Progress Tracker first to set learning goals
2. Self-Assessment Quizzes after each module
3. Common Mistakes Guide when debugging or before interviews
4. Query Explanations for deep dives into complex concepts

Interview Tips

What I've learned works:

• Start simple, build complexity gradually
• Always explain your approach before writing code
• Handle edge cases (NULLs, empty results, etc.)
• Practice timing yourself on realistic problems

Common patterns that come up:

• Basic filtering and joins (modules 1-2)
• Business metrics and aggregations (module 3)
• Complex filtering with subqueries (module 4)
• Data quality and NULL handling (module 5)
• Analytics and rankings (modules 6-7)

Personal Notes

This repo reflects my own SQL learning journey. Started with basic queries and gradually built up to complex analytics. The scenarios are realistic because I pulled them from actual business problems I've encountered.

“Mediocrity is invisible until passion shows up to expose it.”