SQL重复外连接提高效率

Question

问题陈述：

编写查询以返回每个供应商的 2015 年销售信息。我们希望将所有供应商都包括在结果集中，无论他们的产品是否在 2015 年售出。

与前两个问题一样，使用 Sales.Orders 和 Sales.OrderLines 确定销售额。但是，由于我们是从供应商的角度询问此信息，因此您还需要使用表 Warehouse.StockItems 和 Purchasing.Suppliers。

结果集中需要的列是：

SupplierID - 出现在 Purchasing.Suppliers 表中。
SupplierName - 出现在 Purchasing.Suppliers 表中。
OrderCount - 每个供应商的产品订单数量。
Sales - 已下订单的小计，根据 Sales.OrderLines 表的 Quantity 和 UnitPrice 计算。

应对结果进行排序，以使销售额最高的供应商位于顶部。如果两个供应商的销售额相同，则接下来使用顶部计数最高的订单计数。如果两个供应商的销售额和订单数相同，则使用供应商名称升序排列作为最后的决胜局。这将确保一个确定性的结果。

我正在使用WorldWideImporters Microsoft 示例数据库表。我正在尝试为Purchasing.Suppliers 中的每个供应商返回2015 sales information。我在各自的列中返回OrderCount 和Sum of the 2015 sales。我在这里遇到了joins 的问题，因为我必须将Suppliers 连接到Warehouse.StockItems，然后将这些项目连接到具有StockItemID 字段的特定OrderLines。

问题是我通常会加入orders 到orderlines，这样我就可以只过滤orders 和orderlines 中的2015。但是，使用我指定的表结构，似乎我必须将OrderLines 连接到Orders。

所以我所做的就是将join 那些Orders 与OrderLines 一起返回以提供我习惯的结果。这是我的解决方案尝试：

<pre>
SELECT S.SupplierID
      ,S.SupplierName
      ,COUNT(DISTINCT O.OrderID) AS OrderCount
      ,ISNULL(SUM(OLP.Quantity * OLP.UnitPrice), 0.00) AS Sales
FROM Purchasing.Suppliers AS S
LEFT OUTER JOIN Warehouse.StockItems AS W ON S.SupplierID = W.SupplierID
LEFT OUTER JOIN Sales.OrderLines AS OL ON W.StockItemID = OL.StockItemID
LEFT OUTER JOIN Sales.Orders AS O ON OL.OrderID = O.OrderID
                                  AND O.OrderDate BETWEEN '2015-01-01' AND '2015-12-31'
LEFT OUTER JOIN Sales.OrderLines AS OLP ON O.OrderID = OLP.OrderID
GROUP BY S.SupplierID
        ,S.SupplierName
ORDER BY Sales DESC
        ,OrderCount
        ,SupplierName;
</pre>

编辑：

结果： 期待每个supplier 都符合预期，即使没有sales 或orders。我不确定计算出的sales 是否正确，我不确定如何验证。不知道有没有人发现我的查询有问题。

我不知道这是否正确或解决此问题的最有效方法。我确实有一些限制，我只能使用joins，不能使用subqueries、unions 等。

任何有助于理解的帮助将不胜感激。谢谢。

Answer 1

对订单进行基准测试，而不考虑供应商或订单行：

/* query 1 */
SELECT 
       COUNT(*)         AS ordercount
FROM Sales.Orders AS o
WHERE o.OrderDate >= '20150101' AND o.OrderDate < '20160101'

然后对订单行进行基准测试，而不考虑供应商：

/* query 2 */
SELECT
       COUNT(DISTINCT o.OrderID)         AS ordercount
     , SUM(olp.Quantity * olp.UnitPrice) AS sales
FROM Sales.Orders AS o
        INNER JOIN Sales.OrderLines AS olp ON olP.OrderID = o.OrderID
WHERE o.OrderDate >= '20150101' AND o.OrderDate < '20160101'

现在开始引入更多连接，如果值发生变化，则最近的连接有问题：

/* query 3 */
SELECT
       COUNT(DISTINCT o.OrderID)         AS ordercount
     , SUM(olp.Quantity * olp.UnitPrice) AS sales
FROM Sales.Orders AS o
        INNER JOIN Sales.OrderLines AS olp ON olP.OrderID = o.OrderID
        INNER JOIN Warehouse.StockItems AS w ON w.StockItemID = olp.StockItemID
WHERE o.OrderDate >= '20150101' AND o.OrderDate < '20160101'

然后：

/* query 4 */
SELECT
       COUNT(DISTINCT o.OrderID)         AS ordercount
     , SUM(olp.Quantity * olp.UnitPrice) AS sales
FROM Sales.Orders AS o
        INNER JOIN Sales.OrderLines AS olp ON olP.OrderID = o.OrderID
        INNER JOIN Warehouse.StockItems AS w ON w.StockItemID = olp.StockItemID
        INNER JOIN Purchasing.Suppliers s ON s.SupplierID = w.SupplierID
WHERE o.OrderDate >= '20150101' AND o.OrderDate < '20160101'

您当然不需要两次加入订单行，关于左联接，这取决于您要实现的目标，例如：

仅限订单在日期范围内的供应商：

/* query 5 */
SELECT
    s.SupplierID
  , s.SupplierName
  , COUNT(DISTINCT o.OrderID)                       AS ordercount
  , ISNULL(SUM(olp.Quantity * olp.UnitPrice), 0.00) AS sales
FROM Purchasing.Suppliers s
    INNER JOIN Warehouse.StockItems AS w ON s.SupplierID = w.SupplierID
    INNER JOIN Sales.OrderLines AS olp ON w.StockItemID = olp.StockItemID
    INNER JOIN Sales.Orders AS o ON olP.OrderID = o.OrderID
WHERE o.OrderDate >= '20150101' 
    AND o.OrderDate < '20160101' -- note: this is "the next" day
GROUP BY
    s.SupplierID
  , s.SupplierName
ORDER BY
    sales DESC
  , ordercount
  , SupplierName;

所有有库存参考的供应商：

/* query 6 */
SELECT
    s.SupplierID
  , s.SupplierName
  , COUNT(DISTINCT o.OrderID)                       AS ordercount
  , ISNULL(SUM(olp.Quantity * olp.UnitPrice), 0.00) AS sales
FROM Purchasing.Suppliers s
    INNER JOIN Warehouse.StockItems AS w ON s.SupplierID = w.SupplierID
    LEFT JOIN Sales.OrderLines AS olp ON w.StockItemID = olp.StockItemID
    LEFT JOIN Sales.Orders AS o ON olP.OrderID = o.OrderID
                                AND o.OrderDate >= '20150101' 
                                AND o.OrderDate < '20160101' -- note: this is "the next" day
GROUP BY
    s.SupplierID
  , s.SupplierName
ORDER BY
    sales DESC
  , ordercount
  , SupplierName;

每个供应商：

/* query 7 */
SELECT
    s.SupplierID
  , s.SupplierName
  , COUNT(DISTINCT o.OrderID)                       AS ordercount
  , ISNULL(SUM(olp.Quantity * olp.UnitPrice), 0.00) AS sales
FROM Purchasing.Suppliers s
    LEFT JOIN Warehouse.StockItems AS w ON s.SupplierID = w.SupplierID
    LEFT JOIN Sales.OrderLines AS olp ON w.StockItemID = olp.StockItemID
    LEFT JOIN Sales.Orders AS o ON olP.OrderID = o.OrderID
                                AND o.OrderDate >= '20150101' 
                                AND o.OrderDate < '20160101' -- note: this is "the next" day
GROUP BY
    s.SupplierID
  , s.SupplierName
ORDER BY
    sales DESC
  , ordercount
  , SupplierName;

请谨慎使用between作为日期范围，定义日期范围最可靠的方法是使用>=和<，如上图所示，这样无论时间精度如何数据是。 YYYYMMDD 也是 TSQL 中最安全的日期文字格式。