Question

我正在尝试返回在指定日期范围内销售的项目类型的降序列表。

我理解如何获取Item类型的数量并计算它们，但我仍然坚持如何链接到其他表。我将把我的工作代码和psuedo放在一起，我认为它需要完成，任何帮助将不胜感激。

SELECT Item_Type, COUNT(*) as theAmount
FROM  myDB.Item

//LINK order_ID to item 
//WHERE order_ID includes Item_Type

//Link to session
//Find Ses_Date that order was made

GROUP BY Item_Type
ORDER BY theAmount desc
LIMIT 10

我需要三个表来提取数据，显示所需的字段：

Item
PK: Item_ID
Row: Item_Type
FK: Order_ID

Order
PK: Order_ID
FK: Session_ID    

Session
PK: Ses_ID
Row: Ses_date

Answer 1

SELECT   i.Item_Type, COUNT(i.Item_Type) as theAmount
FROM     Item AS i
JOIN     Order AS o ON o.order_ID = i.Order_ID 
JOIN     Session AS s ON s.Ses_ID = o.Session_ID 
WHERE    s.Ses_date between @date1 AND @date2 
GROUP BY i.Item_Type
ORDER BY theAmount DESC
LIMIT    10

Answer 2

SELECT Item_Type, COUNT(*) as theAmount, Session_date
FROM  Item
left outer join Order 
    on Item.Order_id = Order.Order_id
left outer join Session 
    on Order.Session_id = Session.Session_id
GROUP BY Item_Type
ORDER BY theAmount desc
LIMIT 10

以上查询将为您提供结果。

Answer 3

在这种情况下你需要使用LEFT JOIN所以如果你在另一个表上没有匹配的记录，Item_Type的值将为零。

SELECT  a.Item_Type, COUNT(c.Ses_ID) theAmount
FROM    Item a
        LEFT JOIN   `Order` b
            ON  a.Order_ID = b.Order_ID
        LEFT JOIN   `Session` c
            ON b.Session_ID = c.Ses_ID
GROUP BY a.Item_Type
-- ORDER BY theAmount DESC
-- LIMIT 10

MySQL加入表并提取信息以进行检查

3 个答案: