Hello R.A.M.,

it's useful but with the reasonable degree.
You need to undestand that UML should help to get the quick picture of your
project architecture - not bury under "UML<->code corresponding" routine
with a lot of unsignificant details. It must be concise but clear diagrams

Use it as high level instrument, and add details where they are really required
to understand your project - architecture, class/component diagrams, pages
workflow, deployement

R> I am learning to develop ASP.NET applications using SQL databases.
R> How do
R> you think: is UML usefull in designing such applications?

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WBR,
Michael  Nemtsev [C# MVP] :: blog: http://spaces.live.com/laflour

"At times one remains faithful to a cause only because its opponents do not
cease to be insipid." (c) Friedrich Nietzsche