SELECT  *
FROM mdl_user as u
JOIN mdl_role_assignments as ra ON u.id = ra.userid

WHERE
ra.roleid = 5  /* is student */

SELECT * FROM mdl_user_lastaccess

SELECT  *, (SELECT IF ((SELECT count(*) as c FROM mdl_user as u, mdl_user_lastaccess as ula WHERE u.id = ula.userid), 1, 0)) as loggedin
FROM mdl_user as u
JOIN mdl_role_assignments as ra ON u.id = ra.userid

WHERE
ra.roleid = 5