First Quality Gate: Row Count

Concepts covered: paFirstQualityGate, paSqlAssertion

Concepts become useful when applied. The exercise here builds a complete first quality gate: a SQL assertion that the row count for a daily order summary table falls within an expected range. The gate is implemented as a SQL query, the query is run by the orchestrator after the transform finishes, and the gate halts the DAG when the assertion fails. The result is a working quality gate in fewer than thirty lines of code. The exercise is deliberately small. Small gates ship; large gates linger in design review. The discipline of starting with the simplest possible assertion is the same discipline that pays off in software engineering: a working primitive is the foundation that everything more sophisticated builds on top of. Once the first gate is in production and the team has confidence in

About This Interactive Section

This section is part of the Data Quality and Contracts: 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.

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.