# 방법1
update TABLE_A a inner join
            (select A_KEY, count(*) as CNT
             from TABLE_B
             group by A_KEY
            ) b
on (a.A_KEY = b.A_KEY )		 
set a.COUNT = b.CNT
where 1=1
;

# 방법2
update TABLE_A a
set COUNT = (select count(*)
                        from TABLE_B b
                        where b.A_KEY = a.A_KEY
                        group by b.A_KEY
                       ) 
where 1=1
and exists (select 1
                  from TABLE_B b
                  where b.A_KEY = a.A_KEY
                 )
;