Question

I’m using Oracle XE 10g.

Please I beg you to read my question carefully. I have a weird use case for this but please bear with it.

Let’s say I have the following records:

Table person
Name  YearOfBirth
a     null
a     2001
a     2002
b     1990
b     null
c     null
c     2001
c     2009

Basically if I do the following query:

select
  p.Name, max(p.YearOfBirth)
from
  person p
group by
  p.Name

That will give me records with distinct Names and each distinct name will be paired to maximum value of YearOfBirth within its group. In the given example the group where Name=’a’, the maximum YearOfBirth is 2002.

If max() is an aggregate function that returns the maximum value of a column in a given group, is there a function that returns the first value within the group that is not null? Instead of giving me the maximum value, I want the first value you could find as long as it is not null.

Please don’t ask me why I can’t simply use min() or max() instead.

Obviously I can’t use rownum here as some might suggest because doing so will limit the number of groups I could get.

Answer 1

I may be misunderstanding why ROW NUMBER would not work for you. I do not have Oracle, but I did test this in SQL Server, and I believe it provides the results you requested:

WITH soTable AS
(
   SELECT 'a' AS Name, null AS YearOfBirth
   UNION ALL SELECT 'a', 2001
   UNION ALL SELECT 'a', 2002
   UNION ALL SELECT 'b', 1990
   UNION ALL SELECT 'b', null
   UNION ALL SELECT 'b', 1994
   UNION ALL SELECT 'b', 1981
   UNION ALL SELECT 'c', null
   UNION ALL SELECT 'c', 2009
   UNION ALL SELECT 'c', 2001
)
, soTableNoNulls AS
(
   SELECT so.Name, so.YearOfBirth, ROW_NUMBER() OVER (PARTITION BY so.Name ORDER BY so.Name ASC) AS RowNumber
   FROM soTable AS so
   WHERE so.YearOfBirth IS NOT NULL
)
SELECT nn.Name, nn.YearOfBirth
FROM soTableNoNulls AS nn
WHERE nn.RowNumber = 1