如何限制我的sql select只返回max（date）的一行，而不返回多行？

Question

I'm trying to select only one id, which has the latest date column, but this sql query returns multiple rows. How can I select only the latest? MAX() doesn't seem to work

select ra.sourceid as partner1, max(ra.date) from ra_table ra join rap_table rap on parentid=ra.sourceid
where (ra.type = 'Normal' and (ra.name IS NOT NULL or ra.parent IS NOT NULL) and rap.createdate > ra.createdate)
group by ra.sourceid
;

Answer 1

On Oracle 12c or later:

select ra.sourceid as partner1, 
    ra.date
from ra_table ra 
join rap_table rap
    on parentid=ra.sourceid
where (ra.type = 'Normal' 
    and (ra.name IS NOT NULL or ra.parent IS NOT NULL) 
    and rap.createdate > ra.createdate)
order by ra.date desc
fetch first row only;

On Oracle prior to 12c:

select *
from (
    select ra.sourceid as partner1, 
        ra.date
    from ra_table ra 
    join rap_table rap
        on parentid=ra.sourceid
    where (ra.type = 'Normal' 
        and (ra.name IS NOT NULL or ra.parent IS NOT NULL) 
        and rap.createdate > ra.createdate)
    order by ra.date desc
    )
where rownum = 1

Both queries here will return the id and date for the most recent date value among the results. There's no need to aggregate since we're determining what to return with the sort and we only want the one row.

Answer 2

不确定您的情况是否正确，但最后SELECT FIRST 1 ROWS ONLY ”会有所帮助。

Answer 3

You could use MIN( sourceid ) KEEP ( DENSE_RANK FIRST ORDER BY ra.date DESC ) to get the lowest sourceid for the highest date value:

SELECT MIN( ra.sourceid ) KEEP ( DENSE_RANK FIRST ORDER BY ra.date DESC ) as partner1,
       max(ra.date)
from   ra_table ra
       join rap_table rap
       on parentid=ra.sourceid
where  ra.type = 'Normal'
and    (ra.name IS NOT NULL or ra.parent IS NOT NULL)
and    rap.createdate > ra.createdate;

Answer 4

You can use top 1 in your query with descending order:

select Top 1 ra.sourceid as partner1, max(ra.date) from ra_table ra join rap_table rap on parentid=ra.sourceid
where (ra.type = 'Normal' and (ra.name IS NOT NULL or ra.parent IS NOT NULL) and rap.createdate > ra.createdate)
group by ra.sourceid order by max(ra.date) desc

Answer 5

MAX() is a group by function, as such if you ask for more than one column it will give you the max unique combination. As you're using an ID column in your query you are getting uniques always (defeats the purpose of a max).

You should tweak your query a bit:

select TOP 1 ra.sourceid as partner1 FROM ra_table ra 
WHERE ra.date = (SELECT  max(ra.date) 
from 
    ra_table ra 
    join rap_table rap on parentid=ra.sourceid
where 
    (ra.type = 'Normal' and (ra.name IS NOT NULL or ra.parent IS NOT NULL) and rap.createdate > ra.createdate)
group by ra.sourceid)