内部连接的顺序

Question

我正在检查Northwind数据库。有一个名为“发票”的视图相当复杂。

我试图改变原来的sql（拉最后的内部联合起来，与其他内部联合声明合并）。

'官方' 的代码来获取发票：（正确）

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 

我第一次尝试：（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 

我的第二次尝试（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 

两人都返回2155行记录。

这里是表结构的参考：

我的问题是，为什么“我的第一次尝试”是不正确的？ 另外，你认为我的第二次尝试是正确的吗？

感谢

Answer 1

每个ON子句用于指定连接条件，目前尚未有指定的ON最直接的前面JOIN条款。

所以，缩进显示他们是如何匹配起来：

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 

而且最深层的JOIN将首先执行。所以，虽然内最加入似乎是正确的，未来一出（与ON条款orders.OrderID = dbo.[Order Details].OrderID加入orders至Order Details）是不正确 - 我们正在努力加入customer到的结果，以前的加入（或orders和order details ），但是ON子句product.ProductID = dbo.[Order Details].ProductID - 这是错误的，因为我们尚未加入product表。

你可以尝试重新安排他们为：

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 

现在在哪里，至少每个ON子句买卖在范围上表别名方面对每个加盟。

不过，我通常建议（除了复杂的连接）遵循的模式：当每个ON条款保持接近JOIN条款，它实际上规定的条件

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

，多像你的第二次尝试。我可以看到没有理由试图让所有的JOIN发生在FROM的顶部。

Answer 2

在你的第一次尝试，你没有链接托运人顺序表。

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，当您使用连接时，您必须提及将在其上完成连接的两个表的列。

所以当你加入x并加入y时，你必须提及x的列和y的列。否则它将是笛卡尔产品。当您立即进行连接时，您必须提及该列。

在您的第二个查询中，理想的做法是链接所有关联的表1。 可以重新写，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