Selecting the first n rows with Oracle

Question

Please help,I am very confused with this.
For example,lets take a table called "items" which has
ID    NAME    PRICE
1    cup              1.2
2    book         49.99
3    mobile        89.99
20    bike        1250
19    egg            0.8
18    bun           2.5
17    color          2.22
16    tv             310
15    air            0
14    laptop         999.5
13    pack         21.53
12    cd/r           1.2
11    table         198
10    apple         1.05
9    carpet         122.4
8    oracle         19999
7    door           150
6    dollar         1
5    pencil        1.35
Next,say, we want to retrieve the five cheapest items.
select name, price
from items
where rownum < 6
order by price;
NAME                      PRICE
coke                        .78
cup                         1.2
pencil                     1.35
book                      49.99
mobile                    89.99
This is wrong. In the result set above, the item with id no 19 (egg) is missing which only  costs 0.80 (currency units). We get this because
Oracle first retrieves the first five  rows and then orders them by price. This is because of the fact  that we didn't explicitly enough state what we meant with first.
This problem can be solved by using row_number with a option--->below
select name, price
from (
_*select name, price, row_number() over (order by price) r*_
_*from items*_
where r between 1 and 5;
NAME                      PRICE
air                           0
coke                        .78
egg                          .8
dollar                        1
apple                      1.05
**Question is in the above select if i ADD the clause "WHERE rownum <= 6"---The above result set goes wrong again.I dont get the correct order as above.
Please help.Can we have the order by done first and then take 5 records of them.
select name, price
  from (
    select name, price, row_number() over (order by price) r
      *from items WHERE rownum <= 6*
where r between 1 and 5

Answer 1

wrote:user_7000011
Thanks.
*But, what i wanted is  in the inner most select where we select from item --I need to add a where qualification as below.*
select * from
(select name, price, dense_rank() over (ORDER by price desc ) rank from item where rownum &lt;=6
) a
where a.rank&lt;6
Well, the rank limitation is having the same effect in that query, so there's no need to try and put "rownum &lt;=6" in that case.What you need to understand is that "rownum" is a pseudo column that is generated when the results of a query are already determined, but it is applied before any ordering takes place (as the where clause is evaluated before the order by clause).
So if you try and say "where rownum &lt;= 6 order by x", you are under the misunderstanding that it will order the data first and then take the first 6 rows. This isn't the case. It will take the first 6 rows (whatever they may be) and then order those.
In order to order first and then take 6 rows you have to order your data without restriction and then wrap that in a query to restrict it.
Hence...
select x
from tableX
where rownum &lt;= 6
order by xbecomes
select x
from (select x
      from   tableX
      order by x)
where rownum &lt;= 6This ensures that the data is selected and ordered first and then the first 6 rows are taken from that.