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SQL & Databases Masterclass Course

Welcome to the ultimate postgresql tutorial for beginners 2026. Work through interactive sql practice problems with solutions, master our comprehensive sql window functions tutorial modules, and prepare for tough sql interview questions at Google and Amazon entirely in your browser. Claim your course completion XP and build production-ready database mastery today!

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COURSE SYLLABUS
40 Lessons
LESSON WORKSPACEAdvanced

Data Wrangling with SQL: Clean & Transform Your Data

Introduction & Core Concept

Raw enterprise datasets are frequently messy, containing missing metadata, unassigned categories, or empty NULL cells. Data Wrangling is the systematic process of cleaning, structuring, and enriching raw records into pristine reporting assets.

Core Wrangling Tools
  • COALESCE(val1, val2, ...): Evaluates arguments from left to right and returns the very first non-NULL value. It is the gold standard for providing clean default values to empty cells.
  • CASE WHEN: Implements robust if-else logical branching to standardize codes into presentation-ready labels.
    • SELECT COALESCE(email, phone, 'No Contact Info') FROM users;
    Why & Where We Use It
  • Why We Use It: Executive dashboards look unprofessional when displaying blank cells or obscure system codes. Wrangling standardizes data into polished human-readable formats.
  • Where We Use It: Data warehouse ETL staging transformations, preparing clean data exports for machine learning models, and building client-facing reports.
    • Real-World Example

    ShopMart orders ledger contains some orders with missing order dates and raw fulfillment system status codes. The analyst uses COALESCE to inject fallback date strings and CASE WHEN to map raw codes into beautiful emojis and friendly descriptions.

    Best Practices: What to Do & What NOT to Do
  • What to Do: Chain multiple fallback fields inside COALESCE (e.g., COALESCE(work_email, personal_email, 'Unknown')) to establish robust fallback hierarchies.
  • What NOT to Do: Avoid leaving unhandled edge cases in your CASE WHEN branches. Always provide a defensive ELSE fallback clause.
    • Syntax & Pro Tips
    SELECT id, COALESCE(country, 'International') AS country_group FROM users;
    Interactive Sandboxed Terminal (Preloaded DB Schema: SHOPMART)
    SQL Query WorkspaceSQLite v3.45 (WASM Mode)
    QUICK INSERT:
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