Database problems,
and the tell.
The question is in SQL: rows, joins, grouping, ranking. 83 problems in the catalogue carry this tag, from first-week easy to interview hard.
Free to start. Running and submitting a solution never costs credits.
How the pattern actually works.
One mechanism, and the mistake that costs people the problem.
The tell. The question is in SQL: rows, joins, grouping, ranking.
The mechanism. Read the schema before you write anything, decide which join keeps the rows you need, and group only after the set of rows is right. Window functions handle rank and running totals without a self join.
What goes wrong. A join that silently doubles rows. Count before and after, and the bug shows up immediately.
The ones asked most often.
By how often the catalogue records them being set, not by our opinion.
- Consecutive NumbersMediumasked at 3 companies
- Employee BonusEasyasked at 2 companies
- The Number of Users That Are Eligible for DiscountEasyasked at 1 companies
- The Users That Are Eligible for DiscountEasyasked at 1 companies
- Leetflex Banned AccountsMediumasked at 1 companies
- Group Employees of the Same SalaryMediumasked at 1 companies
- Change Null Values in a Table to the Previous ValueMediumasked at 1 companies
- Game Play Analysis IEasyasked at 1 companies
- Game Play Analysis IIEasyasked at 1 companies
- Game Play Analysis IIIMediumasked at 1 companies
Read it, watch it, then solve it.
The order that makes a pattern stick.
Everything in the app, in one place.
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DSA practice
Problems filed under the pattern that solves them, not a sheet.
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Build it inside the app, next to the work you practised.
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One score, and the axis you are weakest on.
tracked dailyOpen
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About database problems.
Free to start, no card.
Start practisingHow do I know a problem is a database problem?
The question is in SQL: rows, joins, grouping, ranking.
How many database problems should I solve?
Enough to stop needing the hint, which is usually ten to fifteen across the levels, not the whole list. The catalogue tags 83, split roughly 33 easy, 40 medium and 10 hard in the sample we show, so work up rather than across.
What goes wrong most often in database problems?
A join that silently doubles rows. Count before and after, and the bug shows up immediately.
Is practising by pattern better than a problem sheet?
A sheet tells you what to solve. A pattern tells you why those problems sit together, which is the part that carries over to the problem you have not seen. That is why the app files every problem under its pattern.