指望有多少次的值跨表發生在這裏

Question

第一篇文章！真的需要一些幫助。

我有建築物的表。在該表中，有多個屬性，其中我在2很感興趣的：

- roof_material 
- additional_roof_material 

在列屋頂材料，有10種不同的屋頂材料，編號爲1-10。

additional_roof_material中列出了相同類型的附加屋頂材料。

我怎麼算，有多少次幢樓roof_material 1，有一個additional_roof_material 1，一個實例？

這就是我想做的事情，只是每種材料：

Select count(*) 
From dbo.40000t where roof_material = 1 and additional_roof_material = 1 

我想這樣做，對每種類型的，所以我得到roof_material 1,2多少次的一個完整的對比， 3,4,5,6,7,8,9,10有additional_roof_material 1,2,3,4,5,6,7,8,9,10。

Answer 1

您可以從表中使用sum()用case語句

select 
    sum((case when roof_material = 1 and additional_roof_material = 1 then 1 else 0 end)) as material_1_1, 
    sum((case when roof_material = 2 and additional_roof_material = 2 then 1 else 0 end)) as material_2_2 
    -- etc... 
from 
    dbo.40000t 

這個查詢有一個隱group by條款得到置換計數 - 因爲我只選擇我集合體並不需要顯式地指定一個group by。

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更新

的要求，在一個評論，如果你想每個置換作爲一個獨特的記錄

select 
    roof_material, 
    additional_roof_material, 
    count(1) as num_instances 
from 
    dbo.40000t 
-- Not sure if you want permutations where roof_material != additional_roof_material or not. 
-- Uncomment these next 2 lines if you do not want those permutations 
--where 
-- roof_material = additional_roof_material 
group by 
    roof_material, 
    additional_roof_material