FILTER and REDUCE

Array manipulation goes beyond reading elements. You can filter, reduce, and aggregate array contents directly in SQL. FILTER for Conditional Rows FILTER with CARDINALITY A common pattern counts how many elements match a condition: This filters the reviews array to include only ratings of 4 or higher, then counts the result. The original reviews array is unchanged. REDUCE for Aggregation REDUCE has four components: the array, an initial accumulator value, a function that combines each element with the accumulator, and a final output function. Calculating Weighted Avg REDUCE enables complex aggregations like weighted averages: The accumulator is a struct with sum and count fields. Each iteration updates both fields. The output function divides sum by count to produce the average.

About This Interactive Section

This section is part of the Complex Data: Advanced 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.