每組選擇第一行

Question

我有一個三列表：A,B,C。

的值是：

+---+-----+----+ 
| A | B | C | 
+---+-----+----+ 
| 1 | -10 | 5 | 
| 1 | 0 | 5 | 
| 1 | 10 | 5 | 
| 2 | 10 | 12 | 
| 2 | 0 | 12 | 
| 3 | -10 | 14 | 
| 4 | 0 | 8 | 
| 4 | 10 | 8 | 
| 5 | 0 | 6 | 
| 5 | 1 | 6 | 
| 5 | -5 | 6 | 
+---+-----+----+ 

如果我第一次訂購由列A，然後列B數據，然後列C（雖然我沒有讓所有列C值每列A值相同）會怎樣我選擇「第一行」每列A？

所以，這將導致：

+---+-----+----+ 
| A | B | C | 
+---+-----+----+ 
| 1 | -10 | 5 | 
| 2 | 0 | 12 | 
| 3 | -10 | 14 | 
| 4 | 0 | 8 | 
| 5 | -5 | 6 | 
+---+-----+----+ 

Answer 1

SELECT a, b, c 
FROM (
     SELECT *, ROW_NUMBER() OVER (PARTITION BY a ORDER BY b, c) rn 
     FROM mytable 
     ) q 
WHERE rn = 1 
ORDER BY 
     a 

或

SELECT mi.* 
FROM (
     SELECT DISTINCT a 
     FROM mytable 
     ) md 
CROSS APPLY 
     (
     SELECT TOP 1 * 
     FROM mytable mi 
     WHERE mi.a = md.a 
     ORDER BY 
       b, c 
     ) mi 
ORDER BY 
     a 

上(a, b, c)創建一個綜合指數的查詢速度更快的工作。

哪一個更有效取決於您的數據分佈。

如果你有幾個不同的值a但每個a內有很多記錄，第二個查詢會更好。

你可以更通過創建一個索引視圖改進：

CREATE VIEW v_mytable_da 
WITH SCHEMABINDING 
AS 
     SELECT a, COUNT_BIG(*) cnt 
     FROM dbo.mytable 
     GROUP BY 
       a 

GO 

CREATE UNIQUE CLUSTERED INDEX 
     pk_vmytableda_a 
ON  v_mytable_da (a) 

GO 

SELECT mi.* 
FROM v_mytable_da md 
CROSS APPLY 
     (
     SELECT TOP 1 * 
     FROM mytable mi 
     WHERE mi.a = md.a 
     ORDER BY 
       b, c 
     ) mi 
ORDER BY 
     a 

Answer 2

這是一個經典的問題，之前

How do I select the first row per group in an SQL Query?

Select first row in each GROUP BY group?

一個好的searchphrase一直問是「選擇第一行的組sql服務器」

Answer 3

SELECT * 
FROM 
    (SELECT *, 
      ROW_NUMBER() OVER (PARTITION BY Dealld 
          ORDER BY Price, Dealld) rn 
    FROM DealOffers) q 
WHERE rn = 1 
ORDER BY Name