sql - 如何在postgres中执行减法

Question

我有以下两个查询，一个是给我总零件作为输出，另一个是给我总分派(dispatch)零件作为输出。我的目标是获得将由“总零件 - 分派(dispatch)零件”给出的总待处理零件。我将如何直接从查询中获得此结果。

代码:

--total parts

select
    dispatches.id,
    count(parts.id) as total_parts
from dispatch_projects,dispatches,parts 
where
    dispatches.id=dispatch_projects.dispatch_id and
    parts.project=dispatch_projects.project and
    parts.ignore=false and
    dispatch_projects.deleted = false and
    dispatches.created > now() - interval '15 days'
group by  dispatches.id
order by dispatches.id

--total dispatched parts




select
    dispatches.id,
    count(parts.id) as dispatched_parts
from dispatch_projects,dispatches,parts,boxes
where
    dispatches.id=dispatch_projects.dispatch_id and
    parts.project=dispatch_projects.project and
    parts.box_id=boxes.id and
    boxes.project=dispatch_projects.project and
    parts.ignore=false and
    dispatch_projects.deleted = false and
    dispatches.created > now() - interval '15 days'
group by  dispatches.id
order by dispatches.id

Answer 1

我们可以在单个查询中获得您想要的结果。

请注意，您的第二个查询与第一个查询的主要区别在于对 boxes 表的额外连接。结果，术语 count(parts.id) 实际上在第一个查询中计算了零件的数量，现在将在第二个查询中计算盒子的数量。但是，我们仍然可以通过取 count(distinct parts.id) 来获取第二个查询中的总零件数。这是有效的，因为它会忽略由于加入 boxes 表而可能发生的部件上的任何重复。考虑以下查询:

SELECT
    d.id,
    COUNT(DISTINCT p.id) - COUNT(p.id) as result
FROM dispatch_projects dp
INNER JOIN dispatches d
    ON d.id = dp.dispatch_id
INNER JOIN parts p
    ON p.project = dp.project
INNER JOIN boxes b
    ON p.box_id = b.id AND
       b.project = dp.project
WHERE
    p.ignore = false AND
    dp.deleted = false AND
    d.created > now() - INTERVAL '15 days'
GROUP BY
    d.id
ORDER BY
    d.id;

请注意，我已将您的逗号分隔连接替换为显式内部连接。这被认为是首选语法。