Oracle: Select at least one record foreach cluster - sql

Question

Look at the following query:
SELECT *
FROM ENI_FLUSSI_HUB c1
WHERE flh_tipo_processo_cod IN ('VA', 'NUOVA_ATT_ENI')
AND rownum < 10
It simply extracts just some VA. I need the extract some VA and some NUOVA_ATT_ENI.
What is the most elegant way to do it?

Answer 1

You can do it like this:
SELECT *
FROM ENI_FLUSSI_HUB c1
WHERE flh_tipo_processo_cod = 'VA'
AND rownum < 5
UNION
SELECT *
FROM ENI_FLUSSI_HUB c1
WHERE flh_tipo_processo_cod = 'NUOVA_ATT_ENI'
AND rownum < 5
Is there aren't any duplicate values, you can use UNION ALL to perform faster:
SELECT *
FROM ENI_FLUSSI_HUB c1
WHERE flh_tipo_processo_cod = 'VA'
AND rownum < 5
UNION ALL
SELECT *
FROM ENI_FLUSSI_HUB c1
WHERE flh_tipo_processo_cod = 'NUOVA_ATT_ENI'
AND rownum < 5
As #DavidAldridge stated, you can always use a view to make this selection.

Answer 2

Here's a nicely overengineered solution:
with
va as (
select rowid ri,
t.*
from eni_flussi_hub t
where flh_tipo_processo_cod = 'VA'
and rownum <= 1),
nuova_att_eni as (
select rowid ri,
t
from eni_flussi_hub t
where flh_tipo_processo_cod = 'NUOVA_ATT_ENI'
and rownum <=1),
the_rest as (
select *
from eni_flussi_hub c1
where flh_tipo_processo_cod in ('VA','NUOVA_ATT_ENI')
and rowid not in (
select ri
from va
union all
select ri
from nuova_att_eni)
and rownum <=9)
select *
from (
select * from va
union all
select * from nuova_att_eni
union all
select * from the_rest
)
where rownum <= 10
/
I think that what it does is return at least one row for each of the two values of flh_tipe_processo_cod, and "lets nature take its course" with the rest.
You'd have to edit the *'s in the main query to avoid trying to include the rowid from the first two subquery factoring clauses.
Here's another, which I think attempts to bring back five of each but will "top up" the required total if less than five are available for either of the two subquery factoring clauses:
with
va as (
select rownum rn,
t.*
from eni_flussi_hub t
where flh_tipo_processo_cod = 'VA'
and rownum <= 10),
nuova_att_eni as (
select rownum rn,
t
from eni_flussi_hub t
where flh_tipo_processo_cod = 'NUOVA_ATT_ENI'
and rownum <=10)
select *
from (
select *
from (select * from va
union all
select * from nuova_att_eni)
order by rn asc
)
where rownum <= 10
/
Enjoy!