COUNT variations (*,col,DISTINCT)

Concepts covered: sqlCount, sqlCountDistinct

COUNT Variations COUNT(*) - Count All Rows COUNT(column) Consider a table where some customers have not provided an email: COUNT(DISTINCT) 1,847 orders came from 892 unique customers. This tells you that some customers placed multiple orders. Practical Applications Comparing COUNT Variants Database Execution

About This Interactive Section

This section is part of the Aggregating: 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 PostgreSQL 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.

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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.