2行を合計し、日付/合計で並べ替えます

このために、結合ではなく選択でデータをフィルタリングする必要があります。

この状態を取り除きます：

AND (month(d.date), year(d.date)) = (month(CURDATE()), year(CURDATE()))

これを選択に追加します：

SUM (CASE WHEN (month(d.date), year(d.date)) = (month(CURDATE()), year(CURDATE())) THEN 1 ELSE 0 END) as monthly

編集：

クエリ全体：

SELECT users.id, users.username, 
  COALESCE(sum(CASE WHEN (month(donations.date), year(donations.date)) = (month(CURDATE()), year(CURDATE())) THEN donations.amount ELSE 0 END), 0) monthly_sum, 
  COALESCE(sum(CASE WHEN (month(donations.date), year(donations.date)) = (month(CURDATE()), year(CURDATE())) THEN 1 ELSE 0 END), 0) monthly_amount, 

  COALESCE(sum(donations.amount), 0) total_sum, 
  count(*) total_amount

from users
left join donations
  on donations.uid = users.id
group by users.id, users.username

http://sqlfiddle.com/#!2/20a974/20/0