This question set covers the SQL topics that come up most often in data engineering interviews at companies running modern data stacks: window functions, CTEs, incremental patterns, performance, and data modelling. Every question includes the full working solution and an explanation of why the answer works. Dataset Setup All questions use these three tables. Run … Read more

Most modern data pipelines ingest semi-structured data — REST API responses, event tracking payloads, webhook bodies, and Kafka messages all arrive as JSON. Knowing how to parse, flatten, and query nested JSON directly in SQL is a fundamental skill for anyone building pipelines on Databricks, Snowflake, or PostgreSQL. This tutorial covers the core JSON operations … Read more

Most data engineers are comfortable with INNER JOIN and LEFT JOIN. But production pipelines regularly require more advanced join patterns — finding records that have no match, comparing rows within the same table, or joining on a range condition rather than an equality. Getting these wrong produces silent data errors or queries that never finish … Read more

A/B testing is not just a product team responsibility. Data engineers build the event tracking pipelines, the experiment assignment tables, and the analysis queries that determine whether a test result is real or just noise. Getting the SQL wrong — double-counting users, mixing pre-experiment data, ignoring statistical significance — produces false results that drive bad … Read more

RFM segmentation is the most practical customer segmentation model for e-commerce and subscription businesses. It classifies every customer on three dimensions: how recently they purchased (Recency), how often they purchase (Frequency), and how much they spend (Monetary). The combination of these three scores places every customer in a segment that tells you exactly how to … Read more

A funnel tracks how users move through a sequence of steps — visit a page, click an item, add to cart, begin checkout, complete purchase. At every step some users drop off. The goal of funnel analysis in SQL is to measure exactly how many users reach each step, the conversion rate between steps, and … Read more

Cohort analysis is one of the most valuable analytical patterns in any business — it answers “are customers we acquired last quarter sticking around better than customers from two years ago?” Without cohort analysis, retention metrics are meaningless averages that hide whether your business is improving or decaying. This tutorial builds a complete cohort analysis … Read more

Window functions are one of the most powerful tools in a data engineer’s SQL toolkit. Unlike aggregate functions that collapse rows into a single result, window functions perform calculations across a set of related rows while keeping every row in the output. This makes them essential for ranking, running totals, comparing rows to previous rows, … Read more

When you are building a data pipeline, duplicate records are one of the most common and damaging data quality problems. A single duplicate row can inflate revenue numbers, double-count customers, or break a UNIQUE constraint on your warehouse table and halt your entire pipeline load. This tutorial walks through three practical SQL methods to detect … Read more

A query that takes 30 seconds in development becomes a 45-minute pipeline blocker in production when it runs against 200 million rows. Query optimization is not optional for data engineers — it is what separates a pipeline that scales from one that needs to be rebuilt six months later. This tutorial covers how to read … Read more