Question

I’m checking for existing of a row in in_fmd, and the ISBN I look up can be the ISBN parameter, or another ISBN in a cross-number table that may or may not have a row.

select count(*) from in_fmd i where (description='GN')     and ( i.isbn in     (         select bwi_isbn from bw_isbn where orig_isbn = ?         union all         select cast(? as varchar) as isbn     ) )  

I don’t actually care about the count of the rows, but rather mere existence of at least one row.

This used to be three separate queries, and I squashed it into one, but I think there’s room for more improvement. It’s PostgreSQL 8.1, if it matters.

Answer 1

Why bother with the UNION ALL

select count(*) from in_fmd i where (description='GN')     and (         i.isbn in (             select bwi_isbn from bw_isbn where orig_isbn = ?         )         or i.isbn = cast(? as varchar)     ) 

I would probably use a LEFT JOIN-style query instead of the IN, but that’s more personal preference:

select count(*) from in_fmd i left join bw_isbn     on bw_isbn.bwi_isbn = i.isbn     and bw_isbn.orig_isbn = ? where (i.description='GN')     and (         bw_isbn.bwi_isbn is not null         or i.isbn = cast(? as varchar)     ) 

The inversion discussed over IM:

SELECT SUM(ct) FROM (     select count(*) as ct     from in_fmd i     inner join bw_isbn         on bw_isbn.bwi_isbn = i.isbn         and bw_isbn.orig_isbn = ?         and i.isbn <> cast(? as varchar)         and i.description = 'GN'      UNION      select count(*) as ct     from in_fmd i     where i.isbn = cast(? as varchar)         and i.description = 'GN' ) AS x