Hi Guys - I am still quite new with this version of Xojo, but am finding it works very well with databases. ;)
Couple points to ponder - Oracle and MS SQL server are both free for development and Small Deployments.
They are several steps beyond MySQL or SQLite in many ways, easy to manage and develop in, and the cost is right. If you find the need to grow, they can grow to just about unlimited size and performance as well. Even deploy the databases securely in the cloud with AWS and/or Azure. Choose Oracle if you intend to deploy your app on MacOS and Linux, because Xojo does not support MSSqlServer on those platforms, so far as I can tell.
Second, if you choose Oracle, be aware XoJo works right now only with 32bit versions of the Oracle client libraries, at least on Windows 8.1, 10, and OSX Yosemite. (Runs fine on 64bit operating systems, just use Oracle InstantClient 32 bit clients.)
This is not an ornerous issue as it won't make any difference unless you are doing a lot of very large transactions - and a customer facing app to store purchase transactions will probably never face that problem. I understand Xojo is working on and very close to releasing a 64bit version of Xojo as well, which will make this consideration just go away.
Last point, neither Oracle or SQL server run on MacOS. Oracle runs great on Windows or Linux, and SQL Server runs only under Windows. That tends to mean you need to run the database server on a virtual machine dedicated to the task, or on a spare machine if you happen to have one. VMs are nice, as running a Linux or Windows VM also allows you to easily test your application on those platforms, if you wish.
p.s. - obviously, the above only makes sense if you are providing a centralized database. If each copy of the app has its own database running on the local machine, SQLlite, Valentina, or even flat files are all good choices to consider. :)