01 / FIELD NOTES
SQL foundations 7 min readOne table. Three answers. Conditional aggregation in SQL.
Build a payment summary with CASE, SUM and COALESCE—and verify its meaning with a tiny dataset.
Foundation
SQL foundations.
Turn messy rows into answers you can explain. Practice grain, NULLs and conditional aggregation.
Start with a guide, work through its example, and explain the result in your own words. Then take the idea into a practice task.
FROM UNDERSTANDING TO DOING
Make the next step a small one.
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Start free practiceSave your own workPractice with browser fixturesClear Free & Pro accessSQL practice uses local DuckDB fixtures. System-design simulations model assumptions; they do not provision real cloud services.