I have a couple apps I've written with Visual Basic 2005 Express that contain SQL databases. I have been experiencing inconsistent behavior during the installation of updates that I would like some clarification on.

If I update an application to a newer version and install it on a client computer, most of the time it seems to recognize it as an update and keeps the database untouched on the client computer.

Sometimes, however, it will install the blank database I have included in the installation and move the client's database to a .pre directory. This is no big deal, I can just move the data back over, but I don't understand why it happens sometimes and not others. I thought maybe it was just if the database in the setup file is newer than the one on the client machine, but it doesn't seem like this is always the case.

And a 3rd scenario. Sometimes the update seems to be viewed as a new install, not an update. It uses the blank database from the setup routine and does not back up or keep any existing database.

I do not change the name of the application or anything between installs other than appropriately incrementing the publish version number.

Please help me understand what is going on. Am I doing something wrong?