SQL Server: Msg 547 Level 16 The ALTER TABLE statement conflicted with the FOREIGN KEY constraint

Aftrer exceuting  

truncate table Department

Msg 547, Level 16, State 0, Line 2
The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_Emp_DepartmentID". The conflict occurred in database "TimeSheet", table "dbo.Department", column 'DepartmentId'.