Oracle Query with Left join and multiple conditions

I have oracle database with Tbl1with fileds fld1, fld2
           Tb9 with fields   Name, Tbl9UID


 I need to look  
      If  Tbl1.Fld1 in ( 1,2,3) then
            if  fld1 ||  fld2 = Tb9.Name  then return  Tbl9UID
      else  if fld1 = Tb9.Name  then return Tbl9UID
      else  return ''


I can do it with case when   statement . But what I am not clear  is   how I can do it when I have to use a left outer join
 it needs to come as one of the multiple  left join for the reasons of another question I asked here
      https://www.experts-exchange.com/questions/29086654/Oracle-query-to-return-extra-fields.html