what to study if you have 2 weeks

friend asked me this today so might as well write it out properly. my answer:

window functions are the single highest-ROI thing to study. they come up everywhere, they trip people up because PARTITION BY feels weird at first, and once you get them they're actually fun. start with ROW_NUMBER and RANK, understand the difference (ties), then DENSE_RANK, then LAG/LEAD. the mental model i give people: think of PARTITION BY as 'GROUP BY that doesn't collapse rows.' that's it. everything else falls out of that.

once windows click, do GROUP BY edge cases. HAVING gets asked a lot and people fumble it because they try to filter on the alias instead of repeating the expression. GROUPING SETS is lower priority but worth knowing exists - i've seen it come up at bigger companies where they want to see you know SQL beyond the basics.

then one system design you can actually walk through cold. not memorized. actually understand it. pick something you've touched at work or can reason about from scratch - batch ingestion pipeline, something that has schema evolution, late arriving data, idempotency. if you can talk through the tradeoffs without prompting you're ahead of 80% of candidates.

the mistake everyone makes in 2 weeks is trying to do everything. cover 20 topics at surface level and you'll get exposed the minute anyone follows up. pick 3 things and go deep. that's actually the answer