Question

我指的是这个数据库：http://www.w3schools.com/sql/trysql.asp?filename=trysql_select_all

我正在尝试完成以下查询：＆＃34;哪个供应商提供大多数产品？＆＃34;

我来到这个查询

select SupplierName, max(count)
from(
    select SupplierName, count(SupplierName) as count
    from 
        Products as p
        inner join 
        Suppliers as s
        on p.SupplierID = s.SupplierID
    group by SupplierName
)

但结果只有一半。

  SupplierName          |   max(count)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
                        |
  Zaanse Snoepfabriek   |   5

值5是正确的，但SupplierName只是内部SQL查询的最后一个值。我总是通过max（count）语句自动提供正确的SupplierName。为什么不喜欢这样？

如果我将查询更改为此，一切正常：

select SupplierName, max(count)
from(
    select SupplierName, count(SupplierName) as count
    from 
        Products as p
        inner join 
        Suppliers as s
        on p.SupplierID = s.SupplierID
    group by SupplierName
) t
where t.count = 
(
    select max(count)
    from(
        select SupplierName, count(SupplierName) as count
        from 
            Products as p
            inner join 
            Suppliers as s
            on p.SupplierID = s.SupplierID
        group by SupplierName
    ) 
)

但是这样我创建了一个全新的表来获取最大值然后将其与t.count进行比较。如果数据库是1,000,000+个记录，事情就会变得严重。

这个问题有更优雅的方法：

计算一些东西，然后得到它的最大值，并获得相关的ID /名称。

Answer 1

你能试试吗？我认为它正在做正确的工作，因为你从表中查询MAX（COUNT），我想它会返回计数最多的值。

select S.SupplierName, R.count
from Suppliers S
LEFT JOIN (
    select SupplierName, count(SupplierName) as count
    from 
        Products as p
        inner join 
        Suppliers as s
        on p.SupplierID = s.SupplierID
    group by SupplierName
) R
ON R.SupplierName = S.SupplierName
ORDER BY S.SupplierName

但是，对于您的原始问题“哪个产品最买？”，我会使用此查询：

SELECT p.productname, od.ProductId, SUM(Quantity)    
FROM [OrderDetails] od
JOIN Products P
ON od.ProductId = P.productId
GROUP BY od.ProductId, P.Productname
ORDER BY SUM(Quantity) DESC

供应商提供材料，与销售商品无关。顾客就是购买者。然而，问题只关注哪种类型的产品以最高的数量销售 - 这可以从订单详细信息表中找到，如果您将其与产品表一起加入，您可以命名所有销售的产品及其销售数量。

<强>更新

对于提供最多产品的供应商，我会使用：

SELECT P.SupplierId, S.SupplierName, COUNT(P.SupplierID) 
FROM Products P
JOIN Suppliers S
ON S.SupplierId = P.SupplierId
GROUP BY P.SupplierId, S.SupplierName
ORDER BY COUNT(P.SupplierId) DESC
LIMIT 1

LIMIT 1（在MSSQL中略有不同 - SELECT TOP 1 ...）将确保您获得最佳结果。但是，如果供应商与供应商品的数量相同，那么您必须将最大数量存储在单独的变量或查询中。正如在W3SCHOOLS托管的网站中使用变量不是要走的路，这是一个可以做同样工作的查询：

SELECT S.SupplierName, t.TotalCount
FROM Suppliers S
JOIN
(   SELECT P.SupplierId, S.SupplierName, COUNT(P.SupplierID) as TotalCount
    FROM Products P
    JOIN Suppliers S
    ON S.SupplierId = P.SupplierId
    GROUP BY P.SupplierId, S.SupplierName) t
ON t.SupplierId = S.SupplierId
JOIN
(   SELECT SupplierId, SupplierName, MAX(TotalCount) as Maximum
    FROM 
    (SELECT P.SupplierId, S.SupplierName, COUNT(P.SupplierID) as TotalCount
    FROM Products P
    JOIN Suppliers S
    ON S.SupplierId = P.SupplierId
    GROUP BY P.SupplierId, S.SupplierName) t2
) t3    
ON t3.Maximum = t.TotalCount
WHERE t.TotalCount = t3.Maximum
ORDER BY t.TotalCount DESC

这与您在原始问题中发布的基本相同，所以它没有多大帮助......：\如果它是在“现实生活”的例子中，您可以引入一个有助于整理的变量查询了一下。基本上你分配我在上面第二个连接中使用的查询结果，你只需在where子句中引用它。

这里有更多变量：http://dev.mysql.com/doc/refman/5.0/en/user-variables.html

SET @max = (SELECT MAX(TotalCount) as Maximum
    FROM 
    (    SELECT P.SupplierId, S.SupplierName, COUNT(P.SupplierID) as TotalCount
        FROM Products P
        JOIN Suppliers S
        ON S.SupplierId = P.SupplierId
        GROUP BY P.SupplierId, S.SupplierName) t);

SELECT S.SupplierName, t.TotalCount
FROM Suppliers S
JOIN
(   SELECT P.SupplierId, S.SupplierName, COUNT(P.SupplierID) as TotalCount
    FROM Products P
    JOIN Suppliers S
    ON S.SupplierId = P.SupplierId
    GROUP BY P.SupplierId, S.SupplierName) t
ON t.SupplierId = S.SupplierId   
WHERE t.TotalCount = @max
ORDER BY t.TotalCount DESC

SQL - 从+ count表中获取组的最大值

1 个答案: