how to convert decimal value in to comma seperated value.

Presentation issues should be handled on the front-end side.
If you change amounts into strings, they wont sort properly, they won't sum any more , ...

you are right byt how can i convert in select statment.
somethinkg like this....
select convert(qty,...) from my table

if you are looking at the result in Query Analyser, you can have it formatted according to your local settings by going to tools->options->connections and select the "use regional settings" option. Then, make sure your reginal settings (from Control Panel) have comma seperator etc.
If you are passing the results on to an application, you are much better off have the application do the formatting, as otherwise you would have to change the data type from decimal to varchar()  etc.

There's no built-in support for this format
instead, most programming languages have formatNumber or formatCurrency functions,
and most reporting tools including excel have standard supprto for htis kind of display format

This could be achieved with a user-defined funciton that would
- cast the number as a string
- find decimal point, if any
- loop every 3 digits left
- build the string in the loop

The function will be ugly and performances will be affected badly

Do you really want this ?

>>The function will be ugly and performances will be affected badly<<
Forgive me for this mis-appreciation

jdlambert has a cool way to get the output you need, and I've learnt something new today ;-)

My memory's so bad, everything I learn is new...  :)