Northwind database is such a database which is available in SQL Server 2000 and which data model is well known to most microsoft based developers. So, when testing or modeling new software framework or architecture, specially from data driven perspective, if we conisder Northwind it reduces sample data modeling, data generation etc overhead and also our architecture or product sample can be easily sharable with other developers.
As relational database deals with three types of relationships, rather using it's whole data model, we can consider only four entities of Northwind database: Employees, Territories, EmployeeTerritories, Region. This sub-set will give us a simpler data model to quickly concentrate on the application architecture or the object model, rather spending time on the analyzing data model and can be used in as much as cases we need to analyze or design.
One-to-One: Employees (EmployeeId, ReportsTo)
Description: one employee reports to another employee.
One-to-Many: Region, Territories (TerritoryId, RegionId)
Description: One region has many territories.
Many-to-Many: Employee, Territories, EmployeeTerritories (EmployeeId, TerritoryId)
Description: One employee belongs to many territories and one territory has many employees.