SUM with CASE 1/0 flag pattern

Question

Accepted Answer

SUM(CASE WHEN condition THEN 1 ELSE 0 END) is a classic conditional count: each matching row contributes 1, each non-matching row contributes 0, and SUM adds them up. Alice and Carol are active (1+1=2), Bob and Dave are inactive (1+1=2). This is equivalent to COUNT(CASE WHEN status='active' THEN 1 END) which exploits the fact that COUNT ignores NULLs - the ELSE 0 vs no ELSE distinction matters only for SUM, not COUNT.