如何查找SQL数据库中物品的平均值

Question

使用Sally的宠物商店数据库我想弄清楚花费的时间超过普通猫的猫。我想它应该开始了这样也许.....

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

，但我得到关于如何获得AVG时，有没有在数据库会告诉我困惑。

以下是访问的关系形式的数据库，所以每个人都可以看到

我认为这将是使用的唯一表是：Animals，SaleAnimal，AnimalOrderItem。

但是有些帮助将是感激，也许在得到了很好的解释这

Answer 1

步骤1：什么是卖猫的平均时间？

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'; 

步骤2：多少只猫花费更长时间才能卖出？

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;