We use Northwind database as the sample database through all the SQL tutorials. Northwind sample database provides you a good table structure and data to allow you to experiment with SQL statements you learn in each tutorial. We also try to make the tutorial as common as possible so you can practice with various Relational Database Management Systems (RDBMS). The sample database is also available in almost common RDBMSs such as MS Access, MS SQL Server, MySQL.
Here is the database diragram of Northwind sample database (click on the diagram image to view it in the full size)