Why do window functions feel like a silver bullet?

When I first started using window functions in BigQuery, it felt like unlocking superpowers. Funnels, rankings, cumulative metrics — all in one elegant query. It feels advanced. It feels correct.

Where does it quietly fall apart?

One wrong PARTITION BY or ORDER BY is enough.

• Users start bleeding into each other
• Sessions lose their boundaries
• Aggregates still look reasonable

That’s the trap — broken logic with believable numbers.

Why GA4 data makes this worse?

GA4 has no strict hierarchy. Users, sessions, events — you define the structure yourself. Window functions don’t protect you from bad assumptions. They amplify them at scale.

What changed my workflow

Before touching window functions, I now force myself to answer one question in plain language: “What is the exact business entity I’m analyzing?” If I can’t answer that clearly, SQL won’t save me.

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