Can this usage of CASE stmt be fine tuned?

Question

Hi,
Need inputs in improving the performance of the below query. (Other forms of rewriting the query)
insert into c(col1, col2, col3)
select b.col1, b.col2,
(case
  when b.col3 is not null and b.col2 = 'x'
  then
  (select col3 from a where a <<join_conditions_1>> b)
  when b.col3 is not null and b.col2 <> 'x'
  then
  (select col3 from a where a  <<join_conditions_2>> b)
  else
  b.col3
  end
from b;
a,b,c are tables
Thanks

Answer 1

Perhaps something like this...
insert into c(col1, col2, col3)
  select b.col1
        ,b.col2
        ,nvl(a.col1,a.col3,b.col3)
  from b
       left outer join a on (   (b.col3 is not null and b.col2 = 'x' and <<join_condition_1>>)
                             or (b.col3 is not null and b.col2 != 'x' and <<join_condition_2>>)
It attempts to (left outer) join to table A using the relevant join conditions, and then if it managed to join, which is tested using the NVL statement, it uses the a.col3 result, otherwise it uses the b.col3 value.