进行数据库测试时需要根据不同场景查询数据，以便验证发现的问题是否为脏数据引起的。记录一下最近常用的查询方法：
1. 查询表中重复数据(id不同，多个字段值相同)
select P1.*
from project as P1, project as P2
where P1.id<>P2.id and P1.ProjectId=P2.ProjectId and P1.ServiceTypeId=P2.ServiceTypeId and P1.Rank=P2.Rank
2.连接多个表数据
select Name, FirstName, LastName, HoursWorked
from employee as E, project as P, assignment as A
where E.EmployeeNumber=A.EmployeeNumber and P.projectID=A.ProjectID
order by P.ProjectID, A.EmployeeNumber
或：
select Name, FirstName, LastName, HoursWorked
from employee as E join assignment as A
on E.EmployeeNumber=A.EmployeeNumber
join project as P
on P.projectID=A.ProjectID
order by P.ProjectID, A.EmployeeNumber