>In this statement : "select * from db where cnum = 'c'".
>
>I want that query return only one record and the query stop after find one record. I don't want that the query scan all the database for it.
>
>What I can append to this query to do it?

Try:

select * TOP 1 from db where cnum = 'c'.

From the theoretical viewpoint your question is a blasphemy (:-)) because the whole theory behind relational databasese and sql is based on sets, therefore a select has always a set as output, which rows are in the output is controled over the where clause. There is not concept like first record, last record and so on. But this is all theory ....