sql查询基于多对多的关系

Question

我有3个表Employee，Project和Assignment。可以将员工分配到多个项目，一个项目可以有多个员工。 Assignment表保持员工与项目之间的多对多关系。我想在sql server中进行sql查询，返回同时处理过多个项目的员工列表。

表格的结构

表员工

EmpId,
EmpName

表项目

ProjId,ProjName,ProjStartDate,ProjEndDate

分配

AssignmentId,ProjId,EmpId,AssStartDate,AssEndDate

Answer 1

Select  EmpName,zt.* from
(
Select a.EmpId,Count(*) as cnt
from Assignment a 
Join Assignment b on a.EmpId=b.EmpId and a.ProjId<>b.ProjId
and 
(
(a.AssStartDate>=b.AssStartDate and a.AssStartDate<=b.AssEndDate) 
or 
(a.AssEndDate>=b.AssStartDate and a.AssEndDate<=b.AssEndDate) 
or
(b.AssStartDate>=a.AssStartDate and b.AssStartDate<=a.AssEndDate) 
or 
(b.AssEndDate>=a.AssStartDate and b.AssEndDate<=a.AssEndDate) 
)
Group by a.EmpId
Having Count(*)>1
) zt
join Employee e on e.EmpId=zt.EmpId

Answer 2

Select a.EmpId from Assignment a join Assignment b on 
a.EmpId = b.EmpId 
and
a.AssStartDate < b.AssEndDate
and
a.AssEndDate > b.AssStartDate