內部連接的順序

Question

我正在檢查Northwind數據庫。有一個名爲「發票」的視圖相當複雜。

我試圖改變原來的sql（拉最後的內部聯合起來，與其他內部聯合聲明合併）。

'官方' 的代碼來獲取發票：（正確）

SELECT * 
FROM   dbo.Shippers  as shipper 
INNER JOIN  dbo.Products  as product 
INNER JOIN  dbo.Employees as employee 
INNER JOIN  dbo.Customers as customer 
INNER JOIN  orders   as orders 
ON customer.CustomerID = orders.CustomerID 
ON employee.EmployeeID = orders.EmployeeID 
INNER JOIN  dbo.[Order Details] 
ON orders.OrderID = dbo.[Order Details].OrderID 
ON product.ProductID = dbo.[Order Details].ProductID 
ON shipper.ShipperID = orders.ShipVia 

我第一次嘗試：（不工作）

SELECT * 
    FROM   dbo.Shippers  as shipper 
    INNER JOIN  dbo.Products  as product 
    INNER JOIN  dbo.Employees as employee 
    INNER JOIN  dbo.Customers as customer 
    INNER JOIN  orders   as orders 
    INNER JOIN  dbo.[Order Details] 
    ON orders.OrderID = dbo.[Order Details].OrderID 
    ON product.ProductID = dbo.[Order Details].ProductID 
    ON shipper.ShipperID = orders.ShipVia 
    ON customer.CustomerID = orders.CustomerID 
    ON employee.EmployeeID = orders.EmployeeID 

我的第二次嘗試（作品）：

select  * 
from Orders as orders 
    inner join Shippers as ships 
     on ships.ShipperID = orders.ShipVia 
    inner join [Order Details] as ods 
     on ods.OrderID = orders.OrderID 
    inner join Products as products 
     on ods.ProductID = products.ProductID 
    inner join Customers as customers 
     on customers.CustomerID = orders.CustomerID 

兩人都返回2155行記錄。

這裏是表結構的參考：

我的問題是，爲什麼「我的第一次嘗試」是不正確的？ 另外，你認爲我的第二次嘗試是正確的嗎？

感謝

Answer 1

每個ON子句用於指定連接條件，目前尚未有指定的ON最直接的前面JOIN條款。

所以，縮進顯示他們是如何匹配起來：

SELECT * 
FROM   dbo.Shippers  as shipper 
INNER JOIN  dbo.Products  as product 
    INNER JOIN  dbo.Employees as employee 
     INNER JOIN  dbo.Customers as customer 
      INNER JOIN  orders   as orders 
       INNER JOIN  dbo.[Order Details] 
       ON orders.OrderID = dbo.[Order Details].OrderID 
      ON product.ProductID = dbo.[Order Details].ProductID 
     ON shipper.ShipperID = orders.ShipVia 
    ON customer.CustomerID = orders.CustomerID 
ON employee.EmployeeID = orders.EmployeeID 

而且最深層的JOIN將首先執行。所以，雖然內最加入似乎是正確的，未來一出（與ON條款orders.OrderID = dbo.[Order Details].OrderID加入orders至Order Details）是不正確 - 我們正在努力加入customer到的結果，以前的加入（或orders和order details ），但是ON子句product.ProductID = dbo.[Order Details].ProductID - 這是錯誤的，因爲我們尚未加入product表。

你可以嘗試重新安排他們爲：

SELECT * 
FROM   dbo.Shippers  as shipper 
INNER JOIN  dbo.Products  as product 
    INNER JOIN  dbo.Employees as employee 
     INNER JOIN  dbo.Customers as customer 
      INNER JOIN  orders   as orders 
       INNER JOIN  dbo.[Order Details] 
       ON orders.OrderID = dbo.[Order Details].OrderID 
      ON customer.CustomerID = orders.CustomerID 
     ON employee.EmployeeID = orders.EmployeeID 
    ON product.ProductID = dbo.[Order Details].ProductID 
ON shipper.ShipperID = orders.ShipVia 

現在在哪裏，至少每個ON子句買賣在範圍上表別名方面對每個加盟。

不過，我通常建議（除了複雜的連接）遵循的模式：當每個ON條款保持接近JOIN條款，它實際上規定的條件

FROM a 
INNER JOIN b 
    ON a.column = b.column 
INNER JOIN c 
    ON a_or_b.column = c.column 
... 

，多像你的第二次嘗試。我可以看到沒有理由試圖讓所有的JOIN發生在FROM的頂部。

Answer 2

在你的第一次嘗試，你沒有鏈接託運人順序表。

SELECT * 
    FROM   dbo.Shippers  as shipper 
    INNER JOIN  dbo.Products  as product 
    INNER JOIN  dbo.Employees as employee 
    INNER JOIN  dbo.Customers as customer 
    INNER JOIN  orders   as orders 
    INNER JOIN  dbo.[Order Details] 
    ON orders.OrderID = dbo.[Order Details].OrderID 
    ON product.ProductID = dbo.[Order Details].ProductID 
    ON shipper.ShipperID = orders.ShipVia 
    ON customer.CustomerID = orders.CustomerID 
    ON employee.EmployeeID = orders.EmployeeID 

第二，當您使用連接時，您必須提及將在其上完成連接的兩個表的列。

所以當你加入x並加入y時，你必須提及x的列和y的列。否則它將是笛卡爾產品。當您立即進行連接時，您必須提及該列。

在您的第二個查詢中，理想的做法是鏈接所有關聯的表1。 可以重新寫，如：

select  * 
from Orders as orders 
    inner join Shippers as ships 
     on orders.ShipVia= ships.ShipperID 
    inner join Customers as customers 
     on orders.CustomerID = customers.CustomerID 
    inner join [Order Details] as ods 
     on orders.OrderID = ods.OrderID 

    inner join Products as products 
     on ods.ProductID = products.ProductID