Query for last n rows please

Question

Hi All,
Can anybody give me a SQL Query which will select last n rows from a table.
guess some time I see this query in this forum but forgot to note it down.
Thanks in Advance
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Answer 1

Hi All,
There is a simple solution, which will be useful at some of the following cases.
1) Find Top( or bottom) <n> rows.
For e.g. Top 5 rows or Bottom 100 rows.
2) Range of rows.
For e.g. Row 11 to row 20. (which is required for displaying the dynamic pages on web search engines)
We need to understand how Oracle processes the query.
SQL>Select rownum, Ename, Sal from Emp;
ROWNUM ENAME SAL
1 A 1000
2 B 800
3 C 3500
But, if we write the query in following manner, check the output to understand Oracle's query processing.
SQL>Select rownum, Ename, Sal from Emp order by Sal;
ROWNUM ENAME SAL
2 B 800
1 A 1000
3 C 3500
The above output makes it clear that the rownum is assigned to the array of output before sorting it.
Therefore, if you have to use the rownum of sorted output, you will have to use the inline query.
SQL>Select rownum, Ename, Sal from (Select Ename, Sal from Emp ) order by Sal;
ROWNUM ENAME SAL
1 B 800
2 A 1000
3 C 3500
The above query can be used, but not for any kind of filter conditions. Please check the tricky part of this filter condition.
SQL>Select rownum, Ename, Sal from (Select Ename, Sal from Emp ) order by Sal
where rownum = 2;
No rows returned.
This is because the rownums are generated first, then the filter is applied for Each row.
Therefore, if the rownum is 1 it is filtered out. Therefore, the second row becomes first row( rownum = 1) and again it gets filtered out.
Therefore, here we need to use one more inline query.
SQL>Select rnum, Ename, Sal from (
Select rownum rnum, Ename, Sal from (Select Ename, Sal from Emp ) order by Sal)
where rnum = 2;
ROWNUM ENAME SAL
2 A 1000
Think, with this above discussion, you can generate any type of query.
Regards.
Yogesh D.