Practice Problem Index

Work these to cement the patterns. Ordered by concept, not difficulty.

Aggregation and GROUP BY

• Count records by status with percent of total
• Multi-metric summary per group with HAVING threshold
• Conditional aggregation with CASE

JOINs

• Multi-table enrichment joins
• Anti-join: LEFT JOIN ... IS NULL vs NOT EXISTS
• Self-join pair problems

Subqueries

• Scalar subquery comparisons against group averages
• EXISTS for multi-condition membership
• IN and NOT IN filtering cases

CTEs

• Waterfall funnel with counts at each step
• Compute -> classify -> summarize pipelines
• Recursive hierarchy traversal with depth/path

Window Functions

• Latest record per group (ROW_NUMBER)
• Top N per group (DENSE_RANK or ROW_NUMBER)
• Running totals
• Month-over-month change with LAG
• Bucketing with NTILE
• Percent of total and cumulative percent

Interview Classics

• Find duplicates
• Nth highest salary
• Consecutive day streaks
• Pivot rows to columns
• Period-over-period growth
• Cohort retention