Question

Lets say a table like this (just for illustration):

KEY  VALUE
  1      A
  2      A    
  3      B
  4      C
  5      A
  6      B
  7      C
  8      A
  9      C

I need a SINGLE SQL to get the key of the 3th (or less) occurrence for EACH value?

VALUE  KEY 
    A    5    (more than 3 occurrences, so it gets the 3th occurrence)  
    B    6    (only 2 occurrences, so it gets the last one)
    C    9 

Update: Oracle 10g

Answer 1

I suspect there is a simpler way to write this without needing 3 layers of nesting. But this should work.

SQL> ed
Wrote file afiedt.buf

  1  with x as (
  2    select 1 key, 'A' val from dual union all
  3    select 2, 'A' from dual union all
  4    select 3, 'B' from dual union all
  5    select 4, 'C' from dual union all
  6    select 5, 'A' from dual union all
  7    select 6, 'B' from dual union all
  8    select 7, 'C' from dual union all
  9    select 8, 'A' from dual union all
 10    select 9, 'C' from dual)
 11  select key,
 12         val
 13    from (select key,
 14                 val,
 15                 rnk,
 16                 max(rnk) over (partition by val) max_rnk
 17            from (select key,
 18                         val,
 19                         rank() over (partition by val order by key) rnk
 20                    from x
 21                 )
 22           where rnk <= 3
 23         )
 24*  where rnk = max_rnk
SQL> /

       KEY V
---------- -
         5 A
         6 B
         9 C