UNION implicitly performs DISTINCT processing, UNION ALL does not.
Besides potential functional difference (i.e. different end result set),
there is a tremendous difference in performance between the two.
I find that in most cases developers really want UNION ALL, but forget to
specify the ALL identifier.
Celebrating 11-Years of SQL Performance Excellence on IBM i5/OS and OS/400
Subject: Re: SQL Question - Merging files or result sets?
I think that UNION will do the trick. Perhaps you could do something like:
selectA = 'SELECT <the fields> FROM fileA WHERE <criteria>'
selectB = 'SELECT <the fields> FROM fileB WHERE <criteria>'
order = ' ORDER BY <ordering fields>'
if (search = both A and B)
query = selectA + ' UNION ' + selectB + order
elseif (search = A only)
query = selectA + order
query = selectB + order
This thread ...
RE: SQL Question - Merging files or result sets?, (continued)
This mailing list archive is Copyright 1997-2020 by midrange.com and David Gibbs as a compilation work. Use of the archive is restricted to research of a business or technical nature. Any other uses are prohibited. Full details are available on our policy page. If you have questions about this, please contact