Hi Edward,
After all, it's working fine now - thanks to your help.
In addition I made the mistake to forget the line THISFORM.REFRESH.
But now I would like to know, what you mean with a 'read-write cursor'.
I thought about it, and your way seems to be the better one indeed. I could also make a table instead of a cursor, and changing always both tables - the original (source) table, and the grid-table. Then I need not to override for every change with a new SELECT.
The disadvantage of the cursor used as grid-source is, that the user feels a short deley between the click on the button and the visible change on the screen. Nevertheless, SQL is amazingly fast.