假设：输出Request Number、Submitter、Assignee。
查询可以简化如下，原因是ASSIGNEE和SUBMITTER不是必需的，USERS加上两个不同角色的别名就足够了。

select r.request_number,
       s.first_name as submitter,
       a.first_name as assignee
  from requests r
  join request_assignee ra
    on r.id = ra.reqeust_id
  join users s
    on r.submitter_id = s.id
  join users a
    on ra.assignee_id = a.id;

当前数据模型：SUBMITTER和ASSIGNEE不提供附加信息。ID列是冗余的。

通过删除SUBMITTER和ASSIGNEE表建议的数据模型。

也许，更改列名称并保持它们一致，然后尝试以下操作

Select r.request_number as Request_Number, u.first_name as Submitter
from requests r, users u
join request_assignee ra on r.id = ra.request_id
join assignee a on u.id = ra.assignee_id
unionall 
Select requests.request_number as Request_Number, users.first_name as Submitter
from requests
Join submitter on requests.submitter_id = submitter.id
Join request_assignee on requests.id = request_assignee.request_id
join users on submitter.id = users.id