The Row Count Surprise
A easy SQL mock interview question on DataDriven. Practice with AI-powered feedback, real code execution, and a hire/no-hire decision.
- 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 your sql solution with real code execution
- Get instant feedback and a hire/no-hire decision