Reading a CASE-Inside-SUM Out Loud

Concepts covered: sqlConditionalAgg

Past correctness, the interviewer probes whether you understand the difference between filtering with WHERE and filtering inside an aggregate. The two produce different shapes; mixing them up is a common bug. This section covers the WHERE vs CASE distinction, the NULL-in-aggregate gotcha, and the divide-by-zero defense for ratios. WHERE filters the rows; CASE filters the contribution The CASE form is the right pattern when the output needs metrics across multiple slices. The WHERE form is the right pattern when the output only needs metrics from one slice (and you want the engine to skip the non-matching rows entirely). Mixing them up: the WHERE filters at the source level, and any subsequent unconditional aggregates see only the filtered rows. State this when the interviewer asks 'why CAS

About This Interactive Section

This section is part of the Conditional Aggregation: Beginner lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

How DataDriven Lessons Work

DataDriven combines four interview rounds (SQL, Python, Data Modeling, Pipeline Architecture) with adaptive difficulty and spaced repetition. Easy problems get harder as you improve. Weak concepts resurface until you master them. Your readiness score tracks progress across every topic interviewers test. Every lesson section ends with problems you solve by writing and running real code, not by picking multiple-choice answers.