In practice this is not... a good way to do this.

Why not? I've been doing exactly that in my custom record editor for the past half-dozen years or so (I always use it with PostgreSQL, though compatibility with MySQL is maintained).

If you can require javascript, then doing an ajax record editor would be better, because you can detect changes through form events rather than the equivalent of "diff -u".(In fact, they is already a variety of self-contained AJAX viewer/updater systems with that feature.)

A much simpler way to do it is to just update every record all the time, even if the user changed nothing, but it's a criminal waste of database resources (particularly if you allow editing a few dozen rows per screen).