Unfortunately, when you delete the DB you lose all of the old alert clips. They are not really clips, but rather pointers to the video file. You still have the video BVR files, but all prior thumbnails and alert pointers are lost.
The few times I have had to delete the DB, I make sure there isn't anything that has happened and I usually wipe the video clean and start fresh. You don't have to do that, but I have found it makes it a little more responsive as the computer isn't trying to write and delete at the same time.