MySQL 合并更新语句：优化 `agreed` 和 `opposed` 字段

UPDATE CommunityAnswer SET agreed = CASE WHEN agreed > 0 AND id = OLD.answerid AND 1 = OLD.opinion THEN agreed - 1 ELSE agreed END, opposed = CASE WHEN opposed > 0 AND id = OLD.answerid AND 0 = OLD.opinion THEN opposed - 1 ELSE opposed END;