Question

我正在检查in_fmd中是否存在一行，而我查找的ISBN可以是ISBN参数，也可以是交叉数字表中可能有也可能没有行的其他ISBN。

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
    )
)

我实际上并不关心行的数量，而只是存在至少一行。

这曾经是三个单独的查询，我把它压成一个，但我认为还有进一步改进的余地。它是PostgreSQL 8.1，如果重要的话。

Answer 1

为什么要打扰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)
    )

我可能会使用LEFT JOIN风格的查询而不是IN，但这更偏向个人：

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)
    )

通过IM讨论的反演：

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

Answer 2

我实际上并不关心行的数量，而只是存在至少一行。

那么如何查询SELECT ... LIMIT 1并检查调用程序是否有一个结果行？

Answer 3

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

Answer 4

除了其他海报所说的，只是改变

选择计数（*）

到

存在（..）

会improve things相当多

Answer 5

select count(*)
from in_fmd i
where description = 'GN'
  and exists (select 1 
              from bwi_isbn 
              where bw_isbn.bwi_isbn = in_fmd.isbn)

如何改进此SQL查询？

5 个答案: