SELECT
COUNT(std.serial_std)
FROM
student_by_course stc
JOIN course crs ON crs.serial_crs = stc.serial_crs
JOIN student std ON stc.serial_std = std.serial_std
WHERE
stc.status_stc = 'ACTIVE'
AND crs.status_crs = 'ACTIVE'
AND std.status_std = 'ACTIVE'
AND crs.serial_lvl = 1