Comparing a Row to Its Neighbors

Concepts covered: sqlSelfJoin

The most common follow-up at this level is 'now do this with a window function instead.' The conversion teaches you what each tool is good at. For comparing each row to the row immediately before it in a sorted order, the window function (LAG) wins on readability and usually performance. For comparing to a row defined by a join condition rather than a row position, the self-join is the only option. The two solutions side-by-side Question: for each transaction, return the previous transaction's amount per account. Both queries produce identical output. The window function is shorter, faster on most engines, and reads as a single pipeline. The self-join version requires a NOT EXISTS subquery (or a correlated max) to pick 'the immediately previous row' from a set of all-previous rows. The sel

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This section is part of the Self-Join: Intermediate 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.

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