No CASE branch matches - what happens?

Question

Accepted Answer

ELSE is optional in CASE. When no WHEN matches and there is no ELSE, SQL returns NULL - not an error and not an empty string. This can silently introduce NULLs that propagate through calculations. For example, SUM(CASE WHEN ... THEN amount END) will ignore non-matching rows (NULL + anything = NULL in aggregation? No - SUM ignores NULLs), but arithmetic like total_price * CASE ... END could go NULL unexpectedly. Explicitly writing ELSE NULL makes the intent clear and helps future readers understand the behavior is intentional.