Question

我有三张表（销售订单，发票，采购订单）

sales_order
------------
so_id (primary key)
item_id (foreign key)
entry_date

invoice
------------
invc_id (primary key)
item_id (foreign key)
entry_date

purchase_order
------------
po_id (primary key)
item_id (foreign key)
entry_date

它们都引用一个中心表（项目）：

item_id (pk)

我正在尝试编写一个sql查询，该查询将返回日期范围内具有活动的所有项目。

这就是我提出的：

select distinct item_id from sales_order where entry_date between ? and ?
union
select distinct item_id from invoice where entry_date between ? and ?
union
select distinct item_id from purchase where entry_date between ? and ?

我认为这是正确的解决方案，但我不确定如何测试它。

问题1：是＆＃34;不同＆＃34;关键字适用于所有语句还是仅适用于每个语句？即，每个查询将返回一个不同的集合，但是当你＆＃34; union＆＃34;它们在一起可以显示重复吗？

问题2：有没有办法返回总（唯一）项目计数（作为单独的查询）？像：

select count(
    select distinct item_id from sales_order where entry_date between ? and ?
    union
    select distinct item_id from invoice where entry_date between ? and ?
    union
    select distinct item_id from purchase where entry_date between ? and ?
)

...

Answer 1

distinct是多余的。我经常写如查询：

select item_id from sales_order where entry_date between ? and ?
union  -- intentionally removing duplicates
select item_id from invoice where entry_date between ? and ?
union
select item_id from purchase where entry_date between ? and ?;

要返回总计数，您可以使用子查询：

select count(*)
from (select item_id from sales_order where entry_date between ? and ?
      union  -- intentionally removing duplicates
      select item_id from invoice where entry_date between ? and ?
      union
      select item_id from purchase where entry_date between ? and ?
     ) i;

从union中选择不同的值

1 个答案: