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Problems

Interview drills. One catalog.

Problems for data engineering interviews: SQL, Python + DSA, and PySpark + Pandas, including the Lakebench SQL, Python + DSA, and PySpark + Pandas study sets. Every problem opens in the same studio. Free covers beginner problems. Pro unlocks intermediate and advanced.

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Beginner (80)

  • Filter Engineering salaries
  • Employee salary aggregates
  • Average salary by department
  • Departments with more than three people
  • Top three salaries
  • Distinct departments
  • Employees without a manager
  • Coalesce null salaries to zero
  • Names starting with A
  • Hired in 2023
  • Employee department names
  • Employees with optional department
  • Customers who never ordered
  • UNION vs UNION ALL counts
  • Salary category buckets
  • Flatten Nested Lists
  • First Occurrence Map
  • Merge Two Sorted Lists
  • Word Frequency Counter
  • Collapse Consecutive Duplicates
  • Top K Words
  • Remove Duplicates Preserving Order
  • Find Intersection
  • Two Sum
  • Find Missing Number
  • Valid Parentheses
  • Longest Consecutive Sequence
  • Group Anagrams
  • Product Except Self
  • Max Subarray Sum
  • Contains Duplicate
  • Valid Anagram
  • Best Time Buy Sell
  • 3Sum
  • Container With Most Water
  • Find All Duplicates
  • Find Disappeared
  • Majority Element
  • Find Duplicate Number
  • Read CSV with Explicit Schema
  • Filter and Select Columns
  • Add Derived Column
  • Handle NULL Values
  • Cast Column Types
  • Explode Array Column
  • Pivot Table
  • Unpivot (Melt) Table
  • Drop Duplicates
  • Sort and Limit
  • Distinct Count
  • GroupBy with Multiple Aggregations
  • Inner Join
  • Left Join
  • Anti-Join
  • Broadcast Join
  • Join with Multiple Conditions
  • Self-Join
  • Read Large CSV in Chunks
  • Merge DataFrames with Different Join Types
  • GroupBy with Multiple Aggregations
  • Pivot Table Creation
  • Handle Missing Values
  • Convert String to Datetime
  • Rolling Window Calculations
  • Most common word in a support ticket
  • Second largest order total
  • Shared tags between two campaigns
  • Rotate a shift schedule
  • Reverse words in a log message
  • Coordinates as immutable records
  • Running sum of daily signups
  • Validate a discount code format
  • Sort products by price then name
  • Even totals from a mixed order list
  • Parse a delimited inventory line
  • Process print jobs in arrival order
  • Time a function's execution
  • Counter for warehouse zone visits
  • Compact a run-length encoded event log
  • Expand a run-length encoded shift log

Intermediate (109)

  • Conditional category revenue
  • Employee and manager names
  • Duplicate customer-product pairs
  • Keep lowest order_id per duplicate pair
  • Second highest salary
  • Second highest salary per department
  • Running total by customer
  • Salary percent of department total
  • Top three products per category
  • Customers with more than two orders
  • Days employed as of 2026-01-01
  • Month-over-month revenue with LAG
  • Year-over-year revenue growth
  • Consecutive login day pairs
  • Gaps in order_id sequence
  • Deduplicate events with ROW_NUMBER
  • RANK vs DENSE_RANK vs ROW_NUMBER
  • Three-day moving average of transactions
  • Highest and lowest salary per department
  • Department as of 2025-06-01
  • Sessionize events with 30-minute gaps
  • Pivot monthly sales to columns
  • Unpivot wide sales to rows
  • Employee hierarchy under Alice
  • Ordered in every month of Q1 2024
  • Salary percentile within department
  • Time since previous login
  • Cumulative orders per product
  • Top three salaries with ties
  • Customers above average spend
  • Trapping Rain Water
  • Valid Palindrome
  • Longest Substring No Repeat
  • Min Window Substring
  • Longest Repeating Char Replace
  • Permutation in String
  • Find All Anagrams
  • Sliding Window Max
  • Longest Substring K Distinct
  • Subarray Sum Equals K
  • Find Anagrams Window
  • Merge Intervals
  • Insert Interval
  • Non-Overlapping Intervals
  • Meeting Rooms
  • Meeting Rooms II
  • Reverse Linked List
  • Merge Two Sorted Lists
  • Linked List Cycle
  • Middle of Linked List
  • Remove Nth From End
  • Reorder List
  • Copy Random List
  • Stack
  • Queue
  • Min Stack
  • Eval RPN
  • Daily Temperatures
  • Row Number for Deduplication
  • Rank vs Dense Rank
  • Running Total
  • Lag and Lead
  • First and Last Value
  • Top N Per Group
  • Percentile Rank
  • Rolling Average
  • Conditional Aggregation
  • GroupBy with Filter (HAVING)
  • Percentage of Total
  • Median Calculation
  • Mode Calculation
  • Cohort Analysis
  • Simple UDF
  • Pandas UDF (Vectorized)
  • UDF with Multiple Columns
  • Avoid UDF When Possible
  • Complex UDF with External Logic
  • UDF Performance Trap
  • Infer Schema vs Explicit Schema
  • Handle Schema Drift
  • Add Missing Columns
  • Rename Columns
  • Drop Columns
  • Validate Data Types
  • Normalize a batch of phone numbers
  • Word frequency ignoring stop words
  • Deduplicate customer records by fuzzy key
  • Group orders by customer and status
  • Longest streak of active days
  • Best 3-day sales window
  • First non-repeating character in a request id
  • Match opening and closing tags
  • Parse a fixed-width mainframe extract
  • Merge two JSON-like config dicts with overrides
  • Extract all email addresses from a nested payload
  • Batch a stream with a size limit
  • Lazily read and filter a large log iterator
  • Safely parse a batch of numeric strings
  • Retry a flaky operation with a cap
  • Model an inventory item with validation
  • Memoize an expensive lookup function
  • Top-K most active accounts
  • Anagram grouping of product SKUs
  • Validate a nested order schema
  • Flatten a directory listing into paths
  • Find the pivot index of a sales list
  • Two-account transfer matching
  • Chunk transactions into daily batches by cutoff time
  • Custom sort by multiple derived keys

Advanced (56)

  • Longest consecutive login streak
  • Usage by plan with point-in-time contracts
  • Never-ordered customers via NOT EXISTS
  • Near-salary pairs in a department
  • Rows remaining after duplicate cleanup
  • Median employee salary
  • Modal salary
  • Missing dates in early January 2024
  • Yearly resetting running total
  • Customers with month-over-month spend decline
  • First-touch attribution
  • Last-touch attribution
  • Overlapping meetings in a room
  • Peak parallel batch workers
  • Sargable rewrite for order_date filter
  • Invert Binary Tree
  • Max Depth
  • Same Tree
  • Subtree
  • LCA BST
  • Level Order
  • Validate BST
  • Number of Islands
  • Clone Graph
  • Course Schedule
  • Connected Components
  • Graph Valid Tree
  • MedianFinder
  • Top K Frequent
  • Merge K Lists
  • Coin Change
  • House Robber
  • Climbing Stairs
  • Repartition vs Coalesce
  • Partition by Column
  • Cache vs Persist
  • When to Cache
  • Broadcast Variable
  • Identify Skew
  • Fix Skew with Salting
  • Small Files Problem
  • Read from Kafka Stream
  • Watermark for Late Data
  • Deduplicate Stream
  • Windowed Aggregation on Stream
  • Write Stream to Delta Lake
  • SCD Type 2 Merge
  • A simple LRU-style access cache
  • Interval merge for maintenance windows
  • Balanced team assignment by workload
  • Weighted round-robin task scheduler
  • Sliding window unique visitor count
  • Detect a cycle in a task dependency map
  • Topological build order for services
  • Context manager for a scoped transaction log
  • Diff two versions of a product catalog
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