從查詢列表中查詢每個類型的SQL查詢最高1值？

Question

我在我的數據庫中有每種產品的不同類型的表格。我想根據該類型爲每個產品拉出最高的1條記錄。這是我當前的查詢：

SELECT 
    dh_type, 
    dh_product 
FROM myProducts 
WHERE dh_productid = '08-BLD4011603S0' 
    AND dh_type IN ('New','Old','Ref') 
ORDER BY dh_createDt DESC 

每種類型可以有多個產品已在不同日期創建。我想爲每種類型拉最高記錄（最新的基於創建日期）。一個新的，一箇舊的和一個參考。我想知道什麼是在SQL中最好和最有效的方法？當前查詢將爲每種類型提取多個記錄，而我只需要一個記錄。

Answer 1

有多種方式可以通過使用窗口功能（如row_number()）來獲得您想要的方式。 row_number函數允許你爲每一行使用一個分區創建一個唯一的數字，在你的情況下，分區是dh_type，然後通過你的dh_createDT命令創建數字。那麼最終的結果將是一個ROW_NUMBER = 1：

select dh_type, 
     dh_product 
from 
(
    SELECT 
     dh_type, 
     dh_product, 
     rn = row_number() over(partition by dh_type order by dh_createDT desc) 
    FROM myProducts 
    WHERE dh_productid = '08-BLD4011603S0' 
     AND dh_type IN ('New','Old','Ref') 
) d 
where rn = 1; 

另一種方式來獲得的結果將是使用子查詢得到max(dh_createDT)每個dh_type和dh_product行，然後加入該回你表：

select 
    dh_type, 
    dh_product 
from myProducts p 
inner join 
(
    select 
    dh_type, 
    dh_product, 
    MaxDate = max(dh_createDT) 
    from myProducts 
    where h_productid = '08-BLD4011603S0' 
    and dh_type IN ('New','Old','Ref') 
    group by dh_type, dh_product 
) p1 
    on p.dh_type = p1.dh_type 
    and p.dh_product = p1.dh_product 
    and p.dh_createDT = p1.MaxDate 
where p.h_productid = '08-BLD4011603S0' 
    and p.dh_type IN ('New','Old','Ref') 

Answer 2

Ollie有一個非常好的答案。然而，如果性能問題，那麼下面可能會更快：

(SELECT TOP (1) dh_type, dh_product 
FROM myProducts 
WHERE dh_productid = '08-BLD4011603S0' AND dh_type = 'New' 
ORDER BY dh_createDt DESC 
) UNION ALL 
(SELECT TOP (1) dh_type, dh_product 
FROM myProducts 
WHERE dh_productid = '08-BLD4011603S0' AND dh_type = 'Old' 
ORDER BY dh_createDt DESC 
) UNION ALL 
(SELECT TOP (1) dh_type, dh_product 
FROM myProducts 
WHERE dh_productid = '08-BLD4011603S0' AND dh_type = 'Ref' 
ORDER BY dh_createDt DESC 
); 

出於性能考慮，你想對myProducts(dh_productid, dh_type, dh_createDt)的索引。您也可以通過dh_product以及索引完全覆蓋查詢。