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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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LESSON WORKSPACEAdvanced

SQL Window Functions: ROW_NUMBER, LAG, LEAD, NTILE

Introduction & Core Concept

Window Functions represent one of the most transformative analytical advancements in SQL architecture. They compute calculations across a specific set of table rows relationally linked to the current row.

Unlike GROUP BY aggregate functions that collapse your rows into a single summary line, window functions preserve your individual row details entirely!

SELECT name, salary, AVG(salary) OVER(PARTITION BY department_id) FROM employees;
Core Window Functions
  • ROW_NUMBER(): Assigns a unique, sequential integer index to rows within specific partitions.
  • LAG(column, offset): Accesses data from preceding rows (perfect for calculating month-over-month revenue growth).
  • LEAD(column, offset): Accesses data from subsequent rows.
  • NTILE(buckets): Divides ordered rows into ranked quartile or percentile groups.
    • Why & Where We Use It
  • Why We Use It: Computing running totals or ranking top 3 employees per department previously required complex self-joins. Window functions solve these tasks elegantly in a single pass.
  • Where We Use It: Financial running balance ledgers, calculating 7-day moving averages, identifying top N products per category, and cohort retention benchmarking.
    • Real-World Example

    StaffCorp management wants to audit compensation by assigning sequential rank indices and quartile bands across employees partitioned by department.

    Best Practices: What to Do & What NOT to Do
  • What to Do: Always specify explicit ORDER BY clauses inside your OVER() window definitions when computing ranking functions like ROW_NUMBER() or LAG().
  • What NOT to Do: Never attempt to place window functions inside your WHERE clause. Window functions are evaluated *after* WHERE filtering occurs!
    • Syntax & Pro Tips
    SELECT name, ROW_NUMBER() OVER(PARTITION BY department_id ORDER BY salary DESC) AS rank FROM employees;
    Interactive Sandboxed Terminal (Preloaded DB Schema: STAFFCORP)
    SQL Query WorkspaceSQLite v3.45 (WASM Mode)
    QUICK INSERT:
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