> Can someone help me on how to delete a secondary file using SQL ?

You can't. Period.

It is not possible using SQL, and it is not possible using any available tool.
(But if you are mad enough you can try to write one; it would not be trivial,
because secondary file can already contain data, that need to be re-allocated
into free space (if there is any) in primary file. Definitely not worth the effort.)
The only solution is to create new database and copy data into it,
which you can easily accomplish by backup/restore.

Why do you add not wanted secondary files in the first place ?

Ivan
http://www.volny.cz/iprenosil/interbase