A little help with transactions, please

Hi all,

This is my first attempt at transactions, and I seem to be missing
something. Briefly, the transaction is this

BEGIN;
    UPDATE table A;
    UPDATE table B;
COMMIT;

I find that the update to table A attempts to produce a duplicate
primary primary key, and synchronisation with the server is lost.

The update to table B then proceeds without the benefit of the
option to rollback the transaction. This seems wrong to me.
I need both updates completed or neither. Have I missed
something?

TIA

Richard A Lough