SQL
The work starts with a question. Before a chart can be trusted, someone has to write the query and check the answer.
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01
Start from the decision
Name what someone has to decide, then write the query that answers it.
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02
Join, filter, aggregate
The everyday mechanics, practiced on messy tables rather than tidy samples.
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03
Check before you ship
Reconcile the totals and sanity-check the counts on your own result.