>I have table merit.dbf having fields Regno, applno, Mathsec, Physec, Chemsec. In my table some records fields value are empty, so I want to extract those records in my table. How to write sql query for the same?

Select * from merit where empty(regno) or empty(applno) or .... && list the rest of the fields in question

If the fields are really blank, you can use isblank() instead of empty()