UPDATE using old value / incrementing values

[Sven Deichmann]

Hello,

I just stumbled upon a problem, I already had before.
But this time it is a performance critical script and this could become 
  a serious problem as I don't want to select the old values before.
Is Valentina really uncapable of accessing the current value within an 
UPDATE?
If I use "UPDATE TrafficSums SET BytesOut=BytesOut+777 WHERE Host LIKE 
'werkbank.com'"
BytesOut for werkbank.com will be 0 afterwards.
Why that?

Thanks in advance,
Sven Deichmann
Werkbank Multimedia GmbH