Question

I have following data

Table1

id   col1    col2       col3
----------------------------------
1    abc   01/01/2012    -
1    abc   01/01/2012    A
2    abc   01/01/2012    -
2    abc   01/02/2012    -
3    abc   01/02/2012    -
3    xyz   01/01/2012    -
4    abc   01/02/2012    -
4    xyz   01/01/2012    -
4    xyz   01/02/2012    -

following is order to evaluate –

if(col1 is false) then evaluate col2 if(col2 is false) then col3:

Col1 - xyz has first preference from all values in this column
col2 - min date
col3 - not '-' or min(col3)

I want to return only one row for each id, if col1 fails go to col2, if this fails then go to col3 condition.
From above table result should be

 id   col1    col2       col3
----------------------------------
 1    abc     01/01/2012  A
 2    abc     01/01/2012  -
 3    xyz     01/01/2012  -
 4    xyz     01/01/2012  -

I tried using dense rank but it didn’t help. I’m not sure how to perform this logic using any available function or sql logic.

for col1 - if more than one row for same code or xyz code then fail
for col2 - if more than one row with same min date then fail 
           [use this only if col1 condition fails]

Answer 1

You can specify many conditions to order by in your analytic function

SELECT *
  FROM (SELECT id,
               col1,
               col2,
               col3,
               dense_rank() over (partition by id
                                      order by (case when col1 = 'xyz' 
                                                     then 1 
                                                     else 0 
                                                 end) desc,
                                               col2 asc,
                                               col3 asc) rnk
          FROM your_table)
 WHERE rnk = 1

I’m assuming that you want dense_rank given that you used the dense_rank tag. You don’t talk about how you want to handle ties or whether ties are even possible, so it’s not clear from the question itself whether you want to use the rank, dense_rank, or row_number analytic functions. If you are only ever fetching the highest ranking row per id, rank and dense_rank will behave identically and will return multiple rows if there are ties for first place. row_number will always return a single row by arbitrarily breaking the tie. If you want to fetch rows other than the first row per id, then you’ll need to think about ties and you’ll get different behavior from rank and dense_rank. If two rows are tied for first, dense_rank will assign the third row a rnk of 2 while rank will assign it a rnk of 3.

This seems to work for the sample data you posted

SQL> ed
Wrote file afiedt.buf

  1  with x as (
  2  select 1 id, 'abc' col1, to_date('01/01/2012', 'MM/DD/YYYY') col2, null col3 from dual union all
  3  select 1 id, 'abc' col1, to_date('01/01/2012', 'MM/DD/YYYY') col2, 'A' col3 from dual union all
  4  select 2 id, 'abc' col1, to_date('01/01/2012', 'MM/DD/YYYY') col2, null col3 from dual union all
  5  select 2 id, 'abc' col1, to_date('01/02/2012', 'MM/DD/YYYY') col2, null col3 from dual union all
  6  select 3 id, 'abc' col1, to_date('01/02/2012', 'MM/DD/YYYY') col2, null col3 from dual union all
  7  select 3 id, 'xyz' col1, to_date('01/01/2012', 'MM/DD/YYYY') col2, null col3 from dual union all
  8  select 4 id, 'abc' col1, to_date('01/02/2012', 'MM/DD/YYYY') col2, null col3 from dual union all
  9  select 4 id, 'xyz' col1, to_date('01/01/2012', 'MM/DD/YYYY') col2, null col3 from dual union all
 10  select 4 id, 'xyz' col1, to_date('01/02/2012', 'MM/DD/YYYY') col2, null col3 from dual
 11  )
 12  SELECT *
 13    FROM (SELECT id,
 14                 col1,
 15                 col2,
 16                 col3,
 17                 dense_rank() over (partition by id
 18                                        order by (case when col1 = 'xyz'
 19                                                       then 1
 20                                                       else 0
 21                                                   end) desc,
 22                                                 col2 asc,
 23                                                 col3 asc) rnk
 24            FROM x)
 25*  WHERE rnk = 1
SQL> /

        ID COL COL2      C        RNK
---------- --- --------- - ----------
         1 abc 01-JAN-12 A          1
         2 abc 01-JAN-12            1
         3 xyz 01-JAN-12            1
         4 xyz 01-JAN-12            1