How to do SQL select top N … in AS400

Question

How do you perform a

Select top N * from as400table

type query against an as400/db2 database

Answer 1

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.