Among the students with a capability below 3, what is the difference of undergraduate students from research postgraduate students?
capability < 3; difference = subtract(count(type = 'UG')), (count(type = 'RPG')); undergraduate students refers to type = 'UG'; research postgraduate students refers to type = 'RPG';
SELECT SUM(CASE WHEN T2.type = 'UG' THEN 1 ELSE 0 END) - SUM(CASE WHEN T2.type = 'RPG' THEN 1 ELSE 0 END) FROM RA AS T1 INNER JOIN student AS T2 ON T1.student_id = T2.student_id WHERE T1.capability < 3