使用DISTINCT關鍵字時，爲什麼不會更改此查詢結果？

Question

爲什麼兩個查詢的結果相同？ （我使用Northwind數據庫）

SELECT  ContactTitle 
     , COUNT(CustomerID) AS NumberOfCustomer 
FROM  dbo.Customers 
WHERE  ContactTitle LIKE '%sales%' 
GROUP BY ContactTitle 
HAVING  COUNT(*) >= 5 
ORDER BY NumberOfCustomer desc 

和

SELECT 
DISTINCT ContactTitle 
     , COUNT(CustomerID) AS NumberOfCustomer 
FROM  dbo.Customers 
WHERE  ContactTitle LIKE '%sales%' 
GROUP BY ContactTitle 
HAVING  COUNT(*) >= 5 
ORDER BY NumberOfCustomer desc 

結果是：

ContactTitle   NumberOfCustomer 
--------------------- ---------------- 
Sales Representative   17 
Sales Manager    11 
Sales Associate    7 
Sales Agent     5 

在我自己的理解，第二個查詢得到不同的標題和計算它的記錄，所以我期待結果將是沒有，因爲每個標題只有記錄計數爲1。

Answer 1

這是查詢執行的工作原理。在您的第二條語句中，DISTINCT不會執行任何其他功能，因爲您的GROUP BY包含相同的列名稱ContactTitle已經爲您執行該操作。

1. FROM 
2. WHERE 
3. GROUP BY <-- You have specified the column `ContactTitle`, 
-- which means the results would be grouped by that column to product unique 
--result. 
4. HAVING 
5. SELECT <-- Adding DISTINCT on ContactTitle column here doesn't make much 
-- difference and it is actually redundant. DISTINCT is applied to the whole 
-- row but the resultset already contains distinct rows grouped by the column 
-- `ContactTitle`. 
6. ORDER BY 

Answer 2

DISTINCT在其他操作之後完成。首先它執行已經使每行不同的GROUP BY，因此DISTINCT是多餘的。

Answer 3

該區別適用於標題和的計數。一旦你的選擇完成計算，它會從中創建不同的列表。

Answer 4

DISTINCT將過濾來自結果集的重複記錄。由於在這種情況下沒有重複記錄，因此DISTINCT不起作用。