How to open a message database(.db) file from Sony Xperia phone in computer?
.db files are not plain-text files, so Notepad is the wrong approach. Those are SQLite databases, so you will have to use some SQLite browser for the task. The linked Wiki page gives you some more details. SQLiteman would be a good choice, and there's even a portable version available. For an alternative, there's also SQLite Browser.