MySQL multiple table joins and multiple data levels - best practice
See if this gives you the correct values:

SELECT t1.id_a, 
        ( SELECT  sum(occ)
            from  t2
            WHERE  t2.id_a = t1.id_fh
            group by  t2.id_a 
        ) AS sum_mc
    FROM xyz AS t1


Note: I am assuming that t1.id_a is unique, so that the GROUP BY is not needed.

(It is confusing to have id_a in both tables, yet not be JOINing on it.)