The Row Count Surprise
An easy SQL mock interview question on DataDriven, with an AI interviewer who asks follow-up questions and a hire or no-hire decision at the end.
- Domain
- SQL
- Difficulty
- easy
- Seniority
- L3
Interview Prompt
The data quality team is auditing how a users-vs-ad_impressions join behaves under the three classic semantics: inner, left, and full outer. Some users were never served an impression, and some impressions don't tie back to a known user. In one result set, return a labeled row for each semantic giving the total number of rows that join produces.
Summary
Same tables. Different handshakes. Wildly different results.
How This Interview Works
- Read the vague prompt (just like a real interview)
- Ask clarifying questions to the AI interviewer
- Write and run your SQL against a real database
- Answer the interviewer's follow-up questions, then get a hire or no-hire decision