window functions

The classic SQL screener at Snowflake and Meta. Everyone can recite the definitions, interviewers are listening for the one scenario where picking the wrong function silently corrupts your results.

A 40-year-old SQL classic that still filters out half of candidates, because of ties, NULLs, and departments with one employee. The interview-proof solution fits in six lines.

The single most-reported live-SQL question in data-platform FDE screens. The pattern is standard, the points are in tie handling, the QUALIFY shortcut, and one aggregation trap most candidates miss.

Every pipeline eventually double-loads data, so every FDE screen eventually asks this. There's one canonical idiom, plus a determinism detail that decides whether your dedupe is rerun-safe.

The hardest 'standard' SQL interview pattern, asked everywhere from Meta to Palantir. There's a three-line trick that turns consecutive runs into a GROUP BY key, once you've seen it, you can't unsee it.

Sessionization is gaps-and-islands with a twist, and it shows up in both SQL screens and Spark rounds at Databricks. The LAG-plus-running-SUM pattern here solves a whole family of interview questions.

A reported Databricks Solutions Architect screen question where the phrase 'handling ties appropriately' is the actual test. The SQL is six lines; the points are in interrogating 'appropriately' and the PySpark follow-up.