separating layers tasks in 3tier architecture

You can do it in both ways. However if you ask me which one I prefer, I would go implementing in BLL. It is not as such good way of implement such kind of validation in DB level. One draw back is propagating the exception from DAL to BLL and then to UI is one step down than propagating from BLL to UI. So without messing around in DB, it is much better implementing in BLL. See thatraja example carefully.

Generally I would put things like this in the business logic layer – after all, it is business logic. However, in the special case of unique constraints on what is presumably a table key, simply setting the primary key (or, if it's not the key column, a unique constraint) on the table will give you this as a bonus side effect so you don't actually need to write any code anywhere. (That assuming you're using a relational database as the data storage layer, of course.)

Firstly, do you want to use any built-in tool? Nextly, if you want a simple validation, then YES, surely that can be done in BLL. Put a simple validation by checking the Data Set you have on that moment and run validation block or function. Stored Procedure can also be used and in that case definitely your application would have a better performance.

Finally, if you can give your BLL and DAL then I can write the code for you also. Obviously, if do not have any other problem giving code.

thanks, so if I do that in BLL, it reduces one level of propagating exceptions, but also it needs to refer to database twice, first to see if it exists,next insert the record. so which one is preferred?!