The Row Count Surprise

A easy SQL interview practice problem on DataDriven. Write and execute real sql code with instant grading.

Domain

SQL

Difficulty

easy

Seniority

L3

Problem

The data quality team is auditing how a users-vs-ad_impressions join behaves under the three classic semantics. Some users were never served an impression, and some impressions don't tie back to a known user. In a single result set, surface three rows labeled inner_join, left_join, and full_outer_join, each with the total row count that combination produces.

Summary

Same tables. Different handshakes. Wildly different results.

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