Recently I needed a test database for MS-SQL. After some search I found the good old NorthWind database that I recalled was a popular MS-SQL and MS-Access demo database. I also found a very nice page that has a download link and detailed instructions on how to get it up and running. This makes it very easy to get started with some demos or tests using MS-SQL. I don't know if NorthWind exists for other SQL dialects, but wouldn't be surprised if it does.
If you want to use the MS-Access Northwind database you can download the old MDB format file off the web. Windows 7 comes with a MS-Access ODBC driver but it only works with the older .mdb file format, not the newer .accdb file format. If you want to use the newer format you will also need to download some files from Microsoft (about 70Mb if I recall) to get to the ODBC driver for the .accdb files. So if you want to do a quick test with Clarion, you should get your hands on the old Access format file and try it.