Elmasri员工数据库 - 确定在不同地点工作的人员

Question

我正在研究Elmasri Employee DB。有关其详细信息，请参阅此文档document。 我已经获得了两个或更多项目及其各自位置的员工：

select w."Essn", p."Plocation"
from "Works_On" w, "Project" p, "Employee" e
where p."Pnumber" = w."Pno" and w."Essn" = e."Ssn"
        and 
        w."Essn" in 
        (select w1."Essn"
        from "Works_On" w1
        group by w1."Essn"
        having count(*) >= 2
        )

结果关系看起来像

我需要确定在两个或更多不同位置工作的员工。因此，987987987和999887777不符合条件，因为他们正在处理两个不同的项目，但位于同一位置。我提出的解决方案是根据员工Essn和Plocation属性对员工进行分组，并确定那些等于一的员工。更新的代码将是：

select base."Essn", base."Plocation"
from
    (select w."Essn", p."Plocation"
    from "Works_On" w, "Project" p, "Employee" e
    where p."Pnumber" = w."Pno" and w."Essn" = e."Ssn"
        and 
        w."Essn" in 
        (select w1."Essn"
        from "Works_On" w1
        group by w1."Essn"
        having count(*) >= 2
        )
    ) as base 

group by base."Essn", base."Plocation"
having count(*) =1

然而，这显然不是真的，因为它会消除33344555 - Stafford。

我的问题是：是否可以从给定的base关系中确定那些在x个不同位置工作的员工？

Answer 1

为了找到在两个不同位置处理两个不同项目的员工，您可能只需要这个查询

  select w."Essn"
  from "Works_On" w
  join "Project" p on p."Pnumber" = w."Pno"
  group by w."Essn"
  having count(distinct w."Pno") > 1 and count(distinct p."Plocation") > 1

并且像这样的查询也应该足够了

  select w."Essn"
  from "Works_On" w
  join "Project" p on p."Pnumber" = w."Pno"
  group by w."Essn"
  having count(distinct p."Plocation") > 1