mysql连接表查询2个值 [英] mysql join table query 2 values

问题描述

我正在尝试在数据库上运行查询以显示具有所选属性的产品

I'm trying to run a query on my database to show products which has the attribute chosen

表1 Products

ID | Product Name
---|-----------------
1    Red Car
2    Blue Car
3    Yellow Car

表2 Attributes

Product ID | Attribute ID
-----------|-----------------
1            3
2            3
3            3
1            4
2            4

例如，我只想显示具有属性3和4设置的产品，它应该只显示红色和蓝色的汽车.但是不是黄色的汽车，因为没有为产品ID 3设置属性集

For example i only want to display products which have attribute 3 and 4 set the it should only show the red and blue car. But not the yellow car as the product as not got a attribute set for product id 3

推荐答案

有很多方法可以解决此问题；最直接的方法可能是使用几个exists子句，或者将attributes表连接两次，但是您也可以使用group by和having子句来实现相同的结果:

There are many ways to solve this; the most straight-forward would probably be to use a couple of exists clauses, or join the attributes table twice, but you could also use group by and having clauses to accomplish the same result:

-- option 1: using multiple exists clauses
select p.id, p.productname
from Products p
where exists (select 1 from Attributes a where p.ID = a.ProductID and a.AttributeID = 3)
  and exists (select 1 from Attributes a where p.ID = a.ProductID and a.AttributeID = 4);

-- option 2: using multiple joins
select p.id, p.productname
from Products p
join Attributes a3 on p.ID = a3.ProductID
join Attributes a4 on p.ID = a4.ProductID
where a3.AttributeID = 3
  and a4.AttributeID = 4;

-- option 3: using aggregate and having
select p.id, p.productname
from Products p
join Attributes a on p.ID = a.ProductID
group by p.id, p.productname
having sum(case when a.AttributeID = 3 then 1 else 0 end) > 0
   and sum(case when a.AttributeID = 4 then 1 else 0 end) > 0;

-- option 4: using having and count
select p.id, p.productname
from Products p
join Attributes a on p.ID = a.ProductID
where a.AttributeID in (3,4)
group by p.id, p.productname
having count(distinct a.attributeid) = 2;

哪种方法最适合您，可能取决于您需要什么结果以及对索引进行索引.

Which way is best for you would probably depend on what result you need and indexes et cetera.

示例SQL小提琴.

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