JOIN survival pocket guide
Recall which join keeps which rows and when to use it.
INNER JOIN versus LEFT JOIN Ask what must survive unmatched. The blanks from the LEFT JOIN make the report messy. Response Those blanks are the unmatched cases; removing them changes the metric population. Filtering NULLs after a LEFT JOIN can erase the denominator. It connects NULL output to business meaning. What does CROSS JOIN create? Every combination of rows from both inputs. Useful for planning grids; dangerous by accident. What is the first check after a surprising join? Count rows before and after each join at the intended grain. The jump points to the multiplying relationship.
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