DataDriven 150

Expanded 150-problem set for deeper interview prep

This study plan is 173 curated data engineering interview problems grouped for structured practice. Each challenge runs real code against live databases or executes Python in a sandboxed environment. You get instant grading, company tier filtering, seniority calibration, and spaced repetition that keeps weak concepts in rotation until they stick.

How to Use This Plan

Start with the easier items to build pattern recognition for the domain.
Move to medium and hard challenges once you can solve easy ones without hints.
After solving a challenge, read the AI discussion phase prompts to pressure-test your solution.
Track your readiness score on your profile and retry items where you struggled.
When you feel ready, launch a full mock interview at /interview for end-to-end simulation.

SQL Challenges (66)

Buyers Who Never Browsed - easy - They bought without ever loading a page.
Double Vision - easy - Before the records move, the ones wearing the same name twice have to surface.
Weekend Warriors - easy - Weekdays vs. weekends. When does the action really happen?
The Dormant Accounts - easy - They are still paying. They stopped showing up.
Average DQ Fail Rate - easy - Average failure rate, table by table.
Category Sales Summary - easy - Category by category. How did they do?
Spending by Account Status - medium - Segment user spending and activity by account status across the platform
Power Users by Session Activity - medium - More sessions. More time. The power users.
30-Day Page View Counts - easy - Thirty days of engagement. Quick snapshot.
Above Average - easy - Products beating the catalog average.
Above Average Interactions - easy - The average user is boring. Who is above?
The Row Count Surprise - easy - Same tables. Different handshakes. Wildly different results.
User Session Roster - easy - Every user paired with their sessions, even users who never logged in
Average Spending by Account Status - medium - Average per-user lifetime spending segmented by account status
The Blind Spot - medium - Pages they haven't discovered yet.
The Ninety-Day Comeback - hard - Everyone shows up once. Who comes back before the quarter ends?
Above Category Average - easy - The category average is one thing. These beat it.
Active API Tokens - easy - Tokens that have actually been used.
Active Campaigns - easy - Which campaigns are earning their keep?
The Token Census - easy - How many tokens are out there?
First Contact - easy - Every pipeline has a first run. This is what it brought back.
Bronze Medal - easy - Only the top few clear the bar.
Cloud Cost Trend Analysis - medium - Cost trends across billing periods.
7-Check Rolling Average - medium - Seven entries hold the trend.
Longest Visit Streaks - hard - Day after day after day. Who kept coming back?
Yesterday's Crown - hard - Every dawn inherits the spending of the day before.
10 Lowest Uptime Services - medium - Ten services at the bottom of the reliability chart.
Bargains and Budget-Busters - hard - Every region has both. Find them.
Time Served - hard - The tokens still in service each carry a history; measure how far it stretches, scope by scope.
Active Users With April Transactions - easy - Active accounts that also opened their wallets. How many?
Activity Histogram - easy - How many users did X things? Build the distribution.
Ad Clickers - easy - Who clicked? What did they spend?
Metric Trend by Department - easy - How each team's numbers moved, year over year.
Provider Cost Change H1 - easy - Cost swings in the first half of the year.
Daily Error Count Change - medium - Errors, trending up or down?
DQ Score Spread - medium - The spread in data quality scores.
Fault Lines - medium - Errors by day and region. Some areas are worse than they appear.
Adopters Before Migration - hard - They used the old feature. Did they ever touch the new one?
Chain of Command - hard - The tree runs deep. Trace every branch back to the top.
The Freshest Record - medium - Duplicates everywhere. Only the most recent version of the truth survives.
Events by Month Across Years - easy - Month by month, year by year. The pattern emerges.
Log Volume by Day of Week - easy - Some days are noisier than others.
Slow Batch Jobs - easy - Promised by noon. Delivered at midnight.
Who We Reached - easy - Monthly reach, campaign by campaign.
The Day-7 Retention Cohort - medium - Day one was promising. Day seven tells the truth.
Early Commit Velocity by Author - medium - How productive was each author during the first year of a repo's CI pipeline
Campaign Click Rate - medium - Among engaged users, which campaigns landed.
Average Accuracy by Framework - medium - Not all frameworks deliver the same accuracy.
The Quiet Alarms - medium - Low severity. High volume. Worth a look.
Quarters Apart - hard - Latency trending up or down? The quarters have the answer.
Campaign Conversion Window - hard - A narrow window between impression and action.
Incident Keyword Messages - hard - Certain words trigger an investigation.
Category-Specific Product Volume - easy - Sum transactions for a specific payment type.
High-Rated In-Stock Percentage - easy - Highly rated and in stock. A rare combo.
The Transaction Breakdown - easy - Multiple time windows. One query. The business wants all of it at once.
Error Severity Buckets - easy - Errors sorted by how much they hurt.
Proof of Presence - medium - Two-factor sent. How many confirmed?
Build Success Rate by Trigger - medium - Which triggers produce green builds?
Alert Response Breakdown - medium - An on-call postmortem asks which services are bleeding alerts nobody acknowledges.
The A/B Verdict - medium - Variant A or Variant B. The conversion numbers pick the winner.
Model Training Completion Rate - medium - How many models finished training?
The Org Chart in Numbers - hard - Headcount by department, sliced by quarter. Every seat accounted for.
Balance of Arms - hard - Every test has two sides. Count who actually landed on each.
Then and Now - hard - Accuracy used to be higher.
Search Algorithm Rating - hard - How good are the search results?
Long Messages - medium - Some commit messages tell a novel.

Python Challenges (62)

The Dominant Signal - easy - Hottest items in the transaction log. Ties included.
The Original Keeper - easy - Clean up duplicate events without losing the timeline.
The Forward Fill - easy - Patch the gaps in a noisy sensor stream.
The Word Mismatch - easy - Some text does not match.
The Mirror Index - easy - Every value remembers who pointed to it.
The Carousel - medium - Every value takes its turn; the wheel keeps spinning.
The Column Shuffle - medium - Rows in, columns out. Number them.
Roots and Branches - hard - The chain of command arrives in pieces. Find where it begins.
The Record Reconciler - medium - Two versions of the same truth.
The Social Graph - easy - Everyone knows someone.
The Sequel Spotter - easy - Spot the sequels hiding in the catalog.
The Numbered Chair - easy - A standing list. Position n holds one entry.
The Column Transformer - easy - Each column gets its function.
The Version Parade - easy - 1.0 before 2.0. Don't let the dots confuse you.
Execution Timer Wrapper - medium - Function wrapped with a timer. Duration captured on exit.
The Chunked Reader - medium - Too big for one payload. Ship it in pieces.
The Lazy Stream - hard - Some slots hold one value, some hold a batch. Level them out.
The Account Manager - easy - Deposits, withdrawals, and the risk of going negative.
The Squeeze - easy - Long stretches of sameness collapse into almost nothing.
The One-Way Street - easy - The longest stretch that never turns around.
The IP Validator - easy - Real and fake, mixed together.
The Log Pulse - easy - Some lines repeat themselves.
The Character Map - easy - Character-level frequency. As a dictionary.
The Event Bucketer - easy - Logs slotted into buckets.
The Status Board - medium - A pile of raw access logs, and two numbers that matter hiding inside the quotes.
The Eviction Policy - medium - Fixed capacity. The key left idle longest is the one that goes.
The Yahtzee Engine - hard - Five dice. Six faces. Score it.
The Frequency Eviction - hard - When storage is tight, something has to go.
The First of Their Kind - easy - When the same record arrives twice, only the first one survives.
The Config Blender - easy - Config collision. The surviving values after a merge.
All the Way Down - easy - Sections nested inside sections. One stream, in order.
Greeting Formatter Class - easy - First impressions are formatted carefully.
The OOP Pillars Exam - medium - Four principles, one class hierarchy - show you know all of them.
The Listeners - medium - Subscribers show up, listen, and sometimes leave.
The Dynamic Container - hard - Grow your own storage before the standard library does it for you.
The Throttle Wall - hard - Stop the abusers. Let the rest through.
The Tail End - easy - Push, pop, peek. The basics that break people.
The Squeeze - easy - aaabbb gets old fast. Shrink it.
Run Length Encoding - easy - AAABBB becomes 3A3B. Compress it.
The Cipher Wheel - easy - Every letter has an alias - you just need the right codebook.
The Encoded Signal - medium - Letters folded into digits, counts hidden in the punctuation.
Reads The Same Both Ways - medium - Somewhere in the noise, a stretch that mirrors itself. Find the longest one.
The Zigzag Encoder - medium - The message snakes its way across the rails.
Common Prefix - hard - They all start the same way. How far?
The Schema Migrator - hard - Old schema in, new schema out.
Open and Shut - easy - Every opener is a promise. The order you close them in decides whether it holds.
The Deep Unpacker - easy - Boxes inside boxes. Eventually you reach the bottom.
The Tree Measurer - easy - How deep does the rabbit hole go?
The Island Counter - medium - Surrounded by water, connected by land - how many separate landmasses?
The Shortest Route - medium - Fewer hops is always better.
The Priority Queue - medium - When two things tie, something has to break the deadlock.
The Coin Vault - medium - Exact change only - and you want to use as few coins as possible.
The Output Peak - hard - One stretch outpaced all the others.
Holding the Center - hard - The middle value keeps moving as new data arrives.
The Triple Alliance - hard - Three numbers, one target.
The Rolling Peak - medium - The sweetest stretch in the sequence.
The Dependency Resolver - medium - Everything depends on everything.
Back to Back - medium - Windows stacked on windows. Find the real spans.
No Vacancy - hard - Every booking needs a room. Find the busiest moment.
The Infection Spread - hard - It starts with one, and then it spreads.
The Unbroken Line - medium - Numbers that flow without interruption.
The Middle Ground - hard - The middle value keeps moving.

Data Modeling Challenges (23)

Where Everyone Was - easy - People move. Sometimes twice in a month. How do you remember where everyone was, and when?
Ghosts in the Ledger - easy - Invoices go out, partial payments trickle in, and some customers are three months overdue.
The No-Show - easy - Every reserved seat ends one of five ways. Build the model that can tell them apart.
Personal Best - easy - Reps, sets, streaks, and personal bests. Gym rats love their stats.
The Endless Thread - medium - Follows, likes, replies to replies. It never stops.
Split Decision - medium - One user, one experiment, one variant. No exceptions.
Trending Dishes Dashboard - medium - What's everyone eating? The answer changes hourly.
Cloud File Storage Metadata Schema - hard - A file is also a folder. A folder is also a file.
Content Search and Discovery Schema - hard - Searchable from every angle. Design it so nothing gets lost.
When the Music Stops - medium - Subscribers are leaving. The data knows why.
E-Commerce Supply Chain Tracking - hard - A package splits, reroutes, and (maybe) arrives.
The Balance Always Reconciles - easy - Money out, payments back. The balance has to be exact.
Toll Road Sensor Analytics - easy - Cars enter, cars exit. Except when they don't.
Ride-Sharing Platform Schema - medium - Riders, drivers, and fares. Everyone takes a cut.
The Retail Blueprint - medium - One business. A thousand transactions. Only one layout survives the analytics layer.
The Table That Lies - medium - Every query comes out wrong. The data is all there.
The Retail Tables That Need a New Home - medium - A working system. Now redesign it so the analysts can actually use it.
The Plan That Changed Twice This Month - medium - Subscribers come, go, downgrade, and share. The schema has to keep up.
Movie Streaming Analytics Schema - medium - They pressed play. What happened next is the whole question.
The Customer Who Changed - hard - She moved. She upgraded. She became someone new. The record has to keep up.
The Churner Who Came Back - hard - They cancelled. They came back. The report has to tell both stories correctly.
The Territory That Keeps Moving - hard - Reps get reassigned. The receipts have to survive.
The Schema That Could Not Answer Back - hard - Forty columns in. Zero useful answers out.

Pipeline Architecture Challenges (22)

Sixty Minutes, Every Hour - medium - Every hour, on the hour. No excuses.
Six Million Rows Before the Market Opens - medium - One massive CSV. Millions of timestamps.
The Meal Kit That Knows You - medium - What they ordered says a lot about what they want next.
The Analysts Cannot Touch Production - medium - Production is the source. Analytics needs its own copy.
The Vendor Who Never Warns You - medium - Every month, something is different. The dashboards have no idea.
Eight Teams, Eight Latencies - medium - Millions of gamers. The architecture decision changes everything.
Every Device Has Its Own Dialect - medium - Three sources. Three formats. Same workout.
Replicate It Without Breaking It - hard - The source changed. The lake needs to know immediately.
Prove the Number Is Right - hard - Bad data in fintech is not just messy. It is expensive.
A Stream All Day and a File at Midnight - hard - Real-time and batch. Same pipeline. No compromises.
The Acquisition Still Taking Bookings - hard - Two systems, two schemas. One truth.
The Decision Before the Door Closes - hard - The window to stop it is smaller than you think.
Five Times the Traffic, Five Times the Bill - hard - Scale up when needed. Do not bankrupt the team.
Thousands of Practices, One Dataset - hard - Patient records in, operational insights out.
End of Day Is Too Late - medium - Every swipe tells a story.
Store, Site, and Distributor - medium - Sales data is piling up. Someone has to make sense of it.
All at Once - medium - Two databases. One direction. No data left behind.
The Bad Row That Broke the Dashboard - medium - Bad records cannot reach the warehouse.
The Provider That Sometimes Sleeps - medium - The models run at dawn. The data has to be there first.
Event System for Multiple Consumers - hard - One event, many hungry consumers.
The Migration That Cannot Break Morning - hard - It all works today. Moving it without losing a single report is the hard part.
The Clock That Runs Two Ways - hard - Nightly batch and live events. One dashboard.