row locking

Hi,

I am sure this is an easy question, but I just don't seem to be able to 
produce an answer to it:
How do you make sure hat a row, that is loaded for editing into a client 
application cannot be altered by another client before it is written back to 
the database?
Do you have to use your own system of extra fields in the database or does 
SQL provide means for locking rows?

Thanks for your help

Sebastian

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Sebastian Boßung - bossung@gmx.de
                http://www.bossung.org
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