MySQL multiple table joins and multiple data levels - best practice I have multiple tables i wish to join up to produce one big query with specific columns summed.
I was having a problem getting the correct summed columns in the final query 
(i believe this was because i was due to data being joined to many lines and summing after the join)

is the correct way to handle this type of problem to join on select statements?
This is the way i achieved it but wasnt sure if this is the correct method?

ie 

select 
    t1.id_a,
    sel_t2.sum_mc
from xyz as t1
    left join
        (select id_a, sum(occ) as sum_mc
        from t2
        group by id_a) as sel_t2
    on t1.id_fh = sel_t2.id_a
group by t1.id_a;
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.)