Panzas2

SELECT DISTINCT(idInstructor) FROM sigsec.`group` g WHERE g.idCec = (SELECT idCec FROM sigsec.`group` g WHERE g.idGroup = 26)
UNION
SELECT DISTINCT(idInstructor) FROM class WHERE idGroup IN ( SELECT DISTINCT(idGroup) FROM sigsec.`group` g WHERE g.idCec = (SELECT idCec FROM sigsec.`group` g WHERE g.idGroup = 26));