Need help in restricting a result set from a UNION in MERGE

Question

Hello,
Would really appreciate if anybody could help me out with the issue I am facing with the below statements (I am new to Oracle ):
merge into table_name_1 p
using
  select p_key, value_1, value_2
  from some_tables
  UNION
  select p_key, value_1, value_2
  from some_tables
  UNION
)t
on (p.p_key = t.p_key)
when matched then
  update table_name_1 with value_1 and value_2
when not matched then
  insert table_name_1 with p_key, value_1, value_2;
Now, the union of all those selects gives me distinct values and it works most of the times but when I get values like below, the merge fails:
p_key-----value_1-----value_2
100-----25-----50
100-----NULL-----50
I browsed the net and understood the reason behind this: the result set becomes ambiguous and merge doesn't know which row to insert first and which one to update.
Now, my requirement is: I could have any of the below scenario/result sets from the union and I need only 1 row per p_key -
result_set_1
p_key-----value_1-----value_2
100-----25-----50 ***************need this row
100-----NULL-----50
100-----NULL-----NULL
result_set_2
p_key-----value_1-----value_2
100-----25-----NULL ***************need this row
100-----NULL-----NULL
result_set_3
p_key-----value_1-----value_2
100-----25-----NULL ***************need this row (p_key = 100)
100-----NULL-----NULL
200-----NULL-----75 ***************need this row (p_key = 200)
200-----NULL-----NULL
300-----90-----95 ***************need this row (p_key = 300)
So, I basically need a way to restrict the values that I will get from the UNION of all those selects to fit the requirement above, hope I was able to explain the issue I am facing.
Any help would be greatly appreciated.
Thanks,
Dpunk

Answer 1

In all cases the goal is to find an order by value that will make the row you want be first.
The query I gave is calculating a priority for each row by adding up values showing whether each column is null or not null. The case statements check whether each column is null and need to be added up to give a total priority value.
Value_1   Value_2   Priority
Not Null  Not Null  2 + 1 = 3
Not Null  Null      2 + 0 = 2
Null      Not Null  0 + 1 = 1
Null      Null      0 + 0 = 0
The priority value ends up being a bitmap showing whether each value is null or not null. I think that reflects my mathematics background.
Another way of getting the same result (suggested to me by your asking why it needs the "+") would be to use two CASE expressions as separate order by items:
select p_key, value_1, value_2 from
(select p_key, value_1, value_2, row_number() over
          (partition by p_key
           order by case when value_1 is null then 0 else 1 end DESC,
                    case when value_2 is null then 0 else 1 end DESC
          ) as rn
  from (your UNION query here)
where rn = 1
A third way is to use a more complex case statement:
select p_key, value_1, value_2 from
(select p_key, value_1, value_2, row_number() over
          (partition by p_key
           order by case when value_1 is NOT null and value_2 is NOT null then 1
                         when value_1 is NOT null and value_2 is     null then 2
                         when value_1 is     null and value_2 is NOT null then 3
                         when value_1 is     null and value_2 is     null then 4
                     end  ASC
          ) as rn
  from (your UNION query here)
where rn = 1