SQL服务器：标记table中的最大百分比值

Question

我有点怀疑，我正在用 Northwind 数据库做一些练习。我正在尝试获取一份报告，按年份显示出货量，以及每个船运公司的出货量总数。它还需要显示该公司完成的出货量百分比，然后需要显示出货量最多的公司（"X" 表示出货量最多的公司，“-”表示其余公司），喜欢代码的注释部分：

IF (Perc > 36, "X", "-")

我已经有了部分代码，我只是在为最后一部分苦苦挣扎。

这是我的：

SELECT 
    /* First we get the years. */
    DISTINCT YEAR(P.OrderDate) AS 'ShipYear',

    /* Now we get the names of the different shipping agencies. */
    (SELECT Shippers.CompanyName
     FROM dbo.Shippers
     WHERE Shippers.ShipperID = P.ShipVia) AS 'Shipper',

    /* The next step is counting the total of shipments *
     *  of that year with that company.                 */
    (SELECT COUNT(C.ShipVia)
     FROM dbo.Orders AS C
     WHERE YEAR(C.OrderDate) = YEAR(P.OrderDate)
       AND C.ShipVia = P.ShipVia) AS 'Shipments',

    /* Now we get it's percentage. */
    (SELECT (COUNT(C.ShipVia) * 100 / 
                        (SELECT COUNT(*)
                         FROM dbo.Orders AS CC
                         WHERE YEAR(CC.OrderDate) = YEAR(P.OrderDate)))
     FROM dbo.Orders AS C
     WHERE YEAR(C.OrderDate) = YEAR(P.OrderDate)
       AND C.ShipVia = P.ShipVia) AS 'Perc'

    --IF (Perc > 36, "X", "-")
FROM 
    [dbo].[Orders] AS P
WHERE 
    P.OrderDate IS NOT NULL
ORDER BY 
    YEAR(P.OrderDate)
GO

这是报告的屏幕截图：

非常感谢您的帮助！

Answer 1

您正在寻找窗口函数，使用 OVER 子句。

您没有指定您使用的是哪个数据库，所以我链接到上面的 Microsoft SQL 服务器。

因为 Perc 列是一个 sub-query，你应该用这样的另一个包裹你的 SELECT 语句：

SELECT 'ShipYear', 'Shipper', 'Shipments', 'Perc'
     , MAX('Perc') OVER (PARTITION BY 'ShipYear') AS `MaxPerc`
  FROM ( // your query goes here
       ) x

这将向您展示它是如何工作的。由于您想要 X 或 -，因此您可以这样做：

SELECT 'ShipYear', 'Shipper', 'Shipments', 'Perc'
     , CASE WHEN 'Perc' = MAX('Perc') OVER (PARTITION BY 'ShipYear')
            THEN 'X'
            ELSE '-'
       END AS 'Best'
  FROM ( // your query goes here
       ) x

SQL服务器：标记table中的最大百分比值

SQL Server : mark the biggest percentage value in a table

sql

max

percentage

northwind