DataDriven 75

Core 75 problems every data professional should know

This prep plan is 75 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 feedback, 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 (24)

Bronze Medal - easy - Only the top few clear the bar.
10 Lowest Uptime Services - medium - Ten services at the bottom of the reliability chart.
7-Check Rolling Average - medium - Seven entries hold the trend.
Bargains and Budget-Busters - medium - Every region has both. Find them.
Cloud Cost Trend Analysis - medium - Cost trends across billing periods.
Daily Error Count Change - medium - Errors, trending up or down?
The Row Count Surprise - easy - Same tables. Different handshakes. Wildly different results.
The Blind Spot - medium - Pages they haven't discovered yet.
The Freshest Record - medium - Duplicates everywhere. Only the most recent version of the truth survives.
Who Stayed - hard - Day after day after day. Who kept coming back?
First Light - medium - Every repo has a scrappy stretch before its first green build. See who was shipping then, and how big their changes were.
The Seventh Day - medium - Day one was promising. Day seven tells the truth.
Error Severity Buckets - easy - Errors sorted by how much they hurt.
Build Success Rate by Trigger - medium - Which triggers produce green builds?
Search Algorithm Rating - hard - How good are the search results?
The Dormant Accounts - easy - They are still paying. They stopped showing up.
Power Users by Session Activity - medium - More sessions. More time. The power users.
Activity Histogram - easy - How many users did X things? Build the distribution.
Provider Cost Change H1 - easy - Cost swings in the first half of the year.
Time Served - hard - The tokens still in service each carry a history; measure how far it stretches, scope by scope.
Against the Clock - easy - Promised by noon. Delivered at midnight.
Long Messages - medium - Some commit messages tell a novel.
Double Vision - easy - Before the records move, the ones wearing the same name twice have to surface.
Spending by Account Status - medium - Segment user spending and activity by account status across the platform

Python Challenges (27)

The Window Cleaner - medium - Keep it fresh, keep it unique.
The Rolling Peak - medium - The sweetest stretch in the sequence.
The Long Run - easy - Say it once, then say how many times it kept saying it.
The Forward Fill - easy - Patch the gaps in a noisy sensor stream.
The Event Bucketer - easy - Logs slotted into buckets.
The Listeners - medium - Subscribers show up, listen, and sometimes leave.
The Mirror Index - easy - Every value remembers who pointed to it.
The Carousel - medium - Every value takes its turn; the wheel keeps spinning.
The Dependency Resolver - medium - Everything depends on everything.
The Throttle Wall - hard - Stop the abusers. Let the rest through.
Back to Back - medium - Windows stacked on windows. Find the real spans.
No Vacancy - hard - Every booking needs a room. Find the busiest moment.
Where the Buck Stops - hard - A pile of reporting lines, no order to them. Somewhere in the bag sits the one with no boss.
The Infection Spread - hard - It starts with one, and then it spreads.
All the Way Down - easy - Sections nested inside sections. One stream, in order.
The Output Peak - hard - One stretch outpaced all the others.
The Unbroken Line - medium - Numbers that flow without interruption.
The Middle Ground - hard - The middle value keeps moving.
The Config Blender - easy - Config collision. The surviving values after a merge.
The Quiet Drift - medium - Two versions of the same truth.
Moving Day - hard - Old schema in, new schema out.
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 a ledger you can walk back.
Execution Timer Wrapper - medium - Function wrapped with a timer. Duration captured on exit.
The Tail End - easy - Push, pop, peek. The basics that break people.
The Eviction Policy - medium - Fixed capacity. The key left idle longest is the one that goes.

Data Modeling Challenges (14)

The Balance Always Reconciles - easy - Money out, payments back. The balance has to be exact.
One Door Each - medium - Every user picks a lane and stays in it. Model the world so the numbers stay honest.
Where Everyone Was - easy - People move. Sometimes twice in a month. How do you remember where everyone was, and when?
The Customer Who Changed - hard - She moved. She upgraded. She became someone new. The record has to keep up.
The Endless Thread - medium - Follows, likes, replies to replies. It never stops.
The Credits Roll - hard - Searchable from every angle. Design it so nothing gets lost.
Personal Best - easy - Reps, sets, streaks, and personal bests. Gym rats love their stats.
When the Music Stops - medium - Subscribers are leaving. The data knows why.
The Retail Blueprint - medium - One business. A thousand transactions. Only one layout survives the analytics layer.
The Long Route - hard - A package splits, reroutes, and (maybe) arrives.
Trending Dishes Dashboard - medium - What's everyone eating? The answer changes hourly.
The Plan That Changed Twice This Month - medium - Subscribers come, go, downgrade, and share. The schema has to keep up.
The Meter's Running - medium - Riders, drivers, and fares. Everyone takes a cut.
The Table That Lies - medium - Every query comes out wrong. The data is all there.

Pipeline Architecture Challenges (10)

The Analysts Cannot Touch Production - medium - Production is the source. Analytics needs its own copy.
The Acquisition Still Taking Bookings - hard - Two systems, two schemas. One truth.
The Provider That Sometimes Sleeps - medium - The models run at dawn. The data has to be there first.
Sixty Minutes, Every Hour - medium - Every hour, on the hour. No excuses.
Replicate It Without Breaking It - hard - The source changed. The lake needs to know immediately.
The Decision Before the Door Closes - hard - The window to stop it is smaller than you think.
Eight Teams, Eight Latencies - medium - Millions of gamers. The architecture decision changes everything.
Five Times the Traffic, Five Times the Bill - hard - Scale up when needed. Do not bankrupt the team.
Store, Site, and Distributor - medium - Sales data is piling up. Someone has to make sense of it.
Thousands of Practices, One Dataset - hard - Patient records in, operational insights out.