Count Student Number in Departments
A medium-tier problem at 60% community acceptance, tagged with Database. Reported in interviews at X and 0 others.
You're staring at a SQL problem that looks straightforward until you realize you need to join tables and aggregate correctly without losing data. Count Student Number in Departments hits that sweet spot where the schema trap catches half the candidates. It's a database problem asking you to count students per department, but the join logic and null handling are where most people stumble. With a 60% acceptance rate and appearing in real OAs, this is exactly the kind of problem that catches you off guard if you haven't thought through your GROUP BY strategy. If this hits your live assessment and you blank on the join cardinality, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
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Count Student Number in Departments is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code.
Get StealthCoderThe trap here is assuming a simple inner join and count will work. Most candidates write a basic GROUP BY and miss that outer joins or missing department records change the output shape entirely. You need to think about whether every department should appear in the result, even if it has zero students, and whether your join could duplicate rows if the cardinality isn't what you think. The real trick is understanding when to use LEFT JOIN, when to COALESCE, and how GROUP BY interacts with NULL values in the grouping column. Database problems reward precision in table relationships and aggregation logic. If you haven't practiced the join patterns and null-handling edge cases under timed pressure, this is a problem where a half-second wrong decision tanks your solution. StealthCoder is the hedge here: if the join logic doesn't click during the OA, a correct solution appears without the proctor seeing it.
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Count Student Number in Departments recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Count Student Number in Departments interview FAQ
Why does my count seem off when I use INNER JOIN?+
Inner join filters out departments with no students entirely. If the problem expects all departments in the result (even with count 0), you need LEFT JOIN to preserve department rows. Test your output against sample data to verify you're not silently dropping rows.
Is this problem still asked in real OAs?+
Yes. Database problems like this appear consistently in technical assessments, especially at larger organizations. The 60% acceptance rate suggests it's a genuine filter, not a warmup. Most candidates miss the join edge case on first attempt.
What's the most common mistake on this problem?+
Assuming INNER JOIN is sufficient and not considering departments that exist but have zero enrolled students. You'll also lose points if your GROUP BY doesn't include the department identifier correctly or if you forget to alias the count column in the SELECT.
How do I know which join to use?+
Read the problem statement carefully. If it says 'count students in each department' without excluding empty departments, assume LEFT JOIN on the department table. Check sample output to confirm whether zero-count rows appear. That's your signal.
What SQL concepts matter most here?+
LEFT JOIN cardinality, GROUP BY aggregation, COUNT() with NULL handling, and result set shape. If you're weak on any of these, this problem will expose it. Run through join behavior on paper before coding in the assessment.
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