大概是这样的：

SELECT users.name, tasks.completed_by month, sum(COALESCE(tasks.cost, 0)) cost_sum
FROM users
JOIN tasks
ON tasks.assignee = users.id
WHERE completed IS NOT NULL AND assignee IS NOT NULL
group by users.name, tasks.completed_by

将 users 加入一个查询，该查询聚合在 tasks 中并返回特定年份每月的总成本：

SELECT u.name, 
       COALESCE(t.cost, 0) AS cost,
       DATE_FORMAT(t.last_day, '%M')
FROM users u
INNER JOIN (
  SELECT assignee, LAST_DAY(completed) last_day, SUM(cost) AS cost
  FROM tasks
  WHERE YEAR(completed) = 2022
  GROUP BY assignee, last_day
) t ON t.assignee = u.id
ORDER BY t.last_day;

不需要检查completed是null还是assignee是null，因为nulls在这里被过滤掉了：

WHERE YEAR(completed) = 2022

这里：

ON t.assignee = u.id