CASE to avoid divide-by-zero

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Accepted Answer

Dividing by zero raises an error in most SQL databases. Wrapping the division in a CASE guards the dangerous branch: when visits is 0, CASE returns NULL without ever evaluating revenue / visits. The short-circuit evaluation of CASE guarantees this - the ELSE branch is only reached when the WHEN condition is FALSE. Returning NULL for undefined ratios is usually more useful than an error, and downstream COALESCE or application logic can handle the NULL as needed.