Select records with few unique columns while choosing not null value over another column in priority

Question

I would like to query a table by getting records unique to field1 and field2 while selecting the row with field4 to have some value rather than null in priority.

e.g. in original table

field1  field2  field3  field4
  1       B       A     (null) 
  1       B       A       2   
  2       C       A       3   
  2       D       A     (null)
  3       D       F       3   
  2       C       A       3   

And what I wish to get from output query is like:

field1  field2  field3  field4
  1       B       A       2   
  2       C       A       3   
  2       D       A     (null)
  3       D       F       3   

Is there any efficient way of SQL table joinning technique / filtering to achieve this? Thanks

P/S - to avoid confusion, the purpose of having distinct of field1 and field2 only is because field3 can be having different values that the chosen row will be based on those row which field4 is not null in priority

e.g.

field1  field2  field3  field4
  1       B       A     (null)   
  1       B       C       2       <- this will be the chosen one

Answer 1

try this:

you Just need to group by the first 3 fields and take the MAX() of filed4

select "field1","field2","field3",max("field4")
from Table1
group by "field1","field2","field3"
order by "field1"

SQL fiddle demo

EDIT:

If you want only field1 and field2 as part of grouping then try this:

select "field1","field2","field3","field4"
from(
select "field1","field2","field3","field4",
        row_number() 
        over(partition by "field1","field2" order by "field4" desc) as rn
from Table1)A
where A.rn=1

SQL Fiddle Demo 2