如何查找SQL數據庫中物品的平均值

Question

使用Sally的寵物商店數據庫我想弄清楚花費的時間超過普通貓的貓。我想它應該開始了這樣也許.....

SELECT AVG (`SaleDate`) AS `Longer to Sale`.... 

，但我得到關於如何獲得AVG時，有沒有在數據庫會告訴我困惑。

以下是訪問的關係形式的數據庫，所以每個人都可以看到

我認爲這將是使用的唯一表是：Animals，SaleAnimal，AnimalOrderItem。

但是有些幫助將是感激，也許在得到了很好的解釋這

Answer 1

步驟1：什麼是賣貓的平均時間？

DECLARE @avg INT; 

SELECT @avg = AVG(DATEDIFF(day, ao.ReceiveDate, s.SaleDate)) 
FROM Animal a 
    INNER JOIN SaleAnimal asale ON (a.AnimalID = asale.AnimalID) 
    INNER JOIN Sale s ON (asale.SaleID = s.SaleID) 
    INNER JOIN AnimalOrderItem aoi ON (a.AnimalID = aoi.AnimalID) 
    INNER JOIN AnimalOrder ao ON (aoi.OrderID = ao.OrderID) 
WHERE a.Breed = 'cat'; 

步驟2：多少隻貓花費更長時間才能賣出？

SELECT COUNT(1) 
FROM Animal a 
    INNER JOIN SaleAnimal asale ON (a.AnimalID = asale.AnimalID) 
    INNER JOIN Sale s ON (asale.SaleID = s.SaleID) 
    INNER JOIN AnimalOrderItem aoi ON (a.AnimalID = aoi.AnimalID) 
    INNER JOIN AnimalOrder ao ON (aoi.OrderID = ao.OrderID) 
WHERE a.Breed = 'cat' 
    AND DATEDIFF(day, ao.ReceiveDate, s.SaleDate) > @avg;