How to do SQL select top N … in AS400

问题:

How do you perform a

Select top N * from as400table

type query against an as400/db2 database

回答1:

Select col1,col2
from 
as400table
where col1='filter'
order by col1
fetch first N row only

Remember to set an ORDER BY clause, because DB2 does not guarantee that the rows returned by FETCH FIRST N ROW ONLY are always the same N.

回答2:

Strictly, there is no equivalent of TOP N in DB2.

SELECT 1 FROM sysibm.sysdummy1
    WHERE EXISTS (SELECT 2 FROM sysibm.sysdummy1)
FETCH FIRST ROW ONLY

compiles and runs, but

SELECT 1 FROM sysibm.sysdummy1
    WHERE EXISTS (SELECT 2 FROM sysibm.sysdummy1 FETCH FIRST ROW ONLY)

will not compile.

TOP N and FETCH FIRST N are not the same. You can only use FETCH FIRST once per query, whereas TOP N can be used in any sub-select.

You can use a window function in a sub-query in order to simulate TOP N:

select *
from (
    select id, row_number()
    over (order by id) as rn
    from testsch.testtbl
) as r
where r.rn < 100 -- This is N rows you are looking for

This will return exactly 99 rows. I tried that in iSeries 7 and it worked.

回答3:

I am only a baby-geek when it comes to IBM--I am a SQL Server guy. But I found the rownumber approach (which I have used successfully in Oracle) did not work in DB2. I used this one:

SELECT
     MYFIELD 
FROM
     "SCHEMANAME"."TABLENAME" 
WHERE
     FILTERCOL1 = 000001 
     AND FILTERCOL2 = 1 
ORDER BY
     MYFIELD DESC FETCH FIRST ROW ONLY

(I ordered descending because I needed the last value.)

Hope this helps. Joey

回答4:

Wouldn't it be easier to limit the result instead? Bellow is in order by date and I take the top result

SELECT banana_equipment_id 
FROM new_banana_equipment 
WHERE banana_code=0000001
ORDER BY banana_date DESC 
LIMIT 1;