> Don't hi-jack threads. Better start a new one.

What do you mean by "hi-jack" ?

> Is there an index on SCHOOL?

Yup.

> Is there an index on PRIORITY?

Yup.

> Use the SQL92 JOIN syntax:
> FROM student_selection s JOIN student_biodata b
> ON (s.student_id=b.student_id)

I've tried this, but it can NOT exclude students who already selected in
another school on higher priority. Can you give more hints ?

Thanks.

-Bee-