您应该发布表的描述和示例数据来说明问题。
在我看来，似乎一条MERGE语句就完成了全部工作（例如，将3个查询合并为1个）：

merge into employee_1 a
  using bonus_1 b
  on (a.employee_id = b.employee_id)
  when matched then update set
    a.salary = a.salary + b.bonus_amount,
    a.last_bonus_date = b.bonus_date;

可以使用MERGE语句并在USING子句中执行聚合：

MERGE INTO employee_1 e
USING (
  SELECT employee_id,
         SUM(bonus_amount) AS total_bonus,
         MAX(bonus_date) AS last_date
  FROM   bonus_1
  GROUP BY employee_id
) b
ON (e.employee_id = b.employee_id)
WHEN MATCHED THEN
  UPDATE
  SET salary          = e.salary + b.total_bonus,
      last_bonus_date = b.last_date;