Just re-writing it, with a bit of correction (not a syntax type):

SELECT ques_id
FROM questions q, categories c
WHERE q.category_id = c.category_id
AND subject = 'Mathematics'
AND category in ('Calculus', 'Geometry', 'Algebra')
ORDER BY RAND()
LIMIT 1

The above query is for the subject 'Mathematics' only. How about for the other subjects? I am try to avoid repeating the above query for the next subject like Astronomy perhaps. I am try to consolidate all these tiny queries for each subject into a single and systematic query.

Thanks! Jay