Question

我有三个表（没有创建数据库）。它们基本相同，但数据略有不同，它们有Company_Id和Category_Id。我还想将类别名称链接到此表，因此我必须加入类别。在加入之前，这工作

Select Distinct CompanyId, Category_ID From Product1 where Product1.CompanyId = 10
union
Select Distinct Distinct CompanyId, Category_ID From Product2 where Product2.CompanyId = 10
union
Select Distinct Distinct CompanyId, Category_ID From Product3 where Product3.CompanyId = 10

根据产品ID，我得到了一份产品和类别的清单。

我现在需要在Category表中添加类别名称。我试过了：

    Select Distinct CompanyId, Category_ID From Product1 where Product1.CompanyId = 10
inner join Category on Category.Id=Product1.Category_ID
    union
    Select Distinct Distinct CompanyId, Category_ID From Product2 where Product2.CompanyId = 10
inner join Category on Category.Id=Product2.Category_ID
    union
    Select Distinct Distinct CompanyId, Category_ID From Product3 where Product3.CompanyId = 10
inner join Category on Category.Id=Product3.Category_ID

但是返回无法绑定多部分标识符“Company.CATEGORY_ID”。

是否只显示包含所有3个表中类别名称的不同项？

（因此，如果product1和product2都有一个Company_ID = 10的产品，并且有两个类别，它只会显示两次）

Answer 1

where位于from子句之后。 join关键字是from的一部分，因此需要先于where：

Select CompanyId, Category_ID
From Product1 inner join
     Category
     on Category.Id = Product1.Category_ID
where Product1.CompanyId = 10
union
Select CompanyId, Category_ID From Product2 
From Product2 inner join
     Category
     on Category.Id = Product1.Category_ID
where Product2.CompanyId = 10
union
Select CompanyId, Category_ID From Product3
From Product3 inner join
     Category
     on Category.Id = Product3.Category_ID
where Product1.CompanyId = 10;

此外，distinct distinct可能会返回错误。您不需要distinct关键字，因为您使用的是union而不是union all。

Answer 2

    Select distinct CompanyId, Category_ID From Product1
inner join Category on Category.Id=Product1.Category_ID where Product1.CompanyId = 10
    union
    Select distinct CompanyId, Category_ID From Product2
inner join Category on Category.Id=Product2.Category_ID  where Product2.CompanyId = 10
    union
    Select distinct CompanyId, Category_ID From Product3
inner join Category on Category.Id=Product3.Category_ID where Product3.CompanyId = 10

这未经过测试，但如果没有拼写错误，它应该可以解决您的问题。

Answer 3

试试这个。我认为首先union加入Category表后会更具可读性和可维护性：

with cte as (
    select distinct CompanyId, Category_ID from Product1 where Product1.CompanyId = 10
    union 
    select distinct CompanyId, Category_ID from Product2 where Product2.CompanyId = 10
    union
    select distinct CompanyId, Category_ID from Product3 where Product3.CompanyId = 10
)
select
   P.CompanyId, P.Category_ID, c.Category_Name
from cte as P
   inner join Category as c on c.Id = P.Category_ID

或者，如果您不喜欢common table expressions，请使用子查询：

select
   P.CompanyId, P.Category_ID, c.Category_Name
from (
    select distinct CompanyId, Category_ID from Product1 where Product1.CompanyId = 10
    union 
    select distinct CompanyId, Category_ID from Product2 where Product2.CompanyId = 10
    union
    select distinct CompanyId, Category_ID from Product3 where Product3.CompanyId = 10
)as P
   inner join Category as c on c.Id = P.Category_ID

使用Joins加入多个表（基本上是同一个表）

3 个答案: