Help with row_number

Question

select * from (
select rownum,col1,col2,col3,
row_number() OVER (order by col1,col2,col3) rn,
col4
from(
select 1 col1,1 col2,1 col3,4 col4 from dual
union all
select 1 col1,1 col2,1 col3,5 col4 from dual
union all
select 1 col1,1 col2,1 col3,6 col4 from dual
where rn=2
i have this query and here in the rn column for inner query i get the below result
ROWNUM     COL1     COL2     COL3     RN     COL4
1     1     1     1     1     4
3     1     1     1     2     6
2     1     1     1     3     5
so from the outer query if i query for rn=2 then in the result i should be getting the second row i.e row with col4=6
but what i get is 3'rd row i.e row with col4=5.
any ideas why?

Answer 1

Roy Mathew wrote:
select * from (
select rownum,col1,col2,col3,
row_number() OVER (order by col1,col2,col3) rn,
col4
from(
select 1 col1,1 col2,1 col3,4 col4 from dual
union all
select 1 col1,1 col2,1 col3,5 col4 from dual
union all
select 1 col1,1 col2,1 col3,6 col4 from dual
where rn=2i have this query and here in the rn column for inner query i get the below result
ROWNUM     COL1     COL2     COL3     RN     COL4
1     1     1     1     1     4
3     1     1     1     2     6
2     1     1     1     3     5so from the outer query if i query for rn=2 then in the result i should be getting the second row i.e row with col4=6
but what i get is 3'rd row i.e row with col4=5.
any ideas why?Because, when you are ordering over col1, col2 and col3 and all those values are identical, the order is essentially random. When you added your "where rn=2" clause to your query SQL processed things slightly differently and the randomness of the rows meant that you got a different row from what you expected, but even that is random.
Data can only be put in a specific order if there is some value to the data that determines that order. If the data is all the same, then it's essentially random, even if you do appear to keep getting the same results (that's just the way the data has been cached and the way the optimiser has processed it the same each time). Changing the query changes the way it's been processed.