查询不返回任何记录

Question

您好，我正在为采购订单制定这些订单可以由某个用户订购，然后由另一个用户接收。

所以我创建了下面的模式。

模式

的问题是当PurchaseOrders和Deliveries表UserID列具有查询返回任何记录不同的值。

查询

SELECT 
    dbo.Users.FirstName, 
    dbo.Users.LastName, 
    dbo.PurchaseOrders.PurchaseOrderDate, 
    dbo.Deliveries.ExpectedDeliveryDate, 
    dbo.Deliveries.ActualDeliveryDate 
FROM dbo.PurchaseOrders 
INNER JOIN dbo.Users 
    ON dbo.PurchaseOrders.UserID = dbo.Users.UserID 
INNER JOIN dbo.Deliveries 
    ON dbo.PurchaseOrders.PurchaseOrderID = dbo.Deliveries.PurchaseOrderID 
    AND dbo.Users.UserID = dbo.PurchaseOrders.UserID 
    AND dbo.Users.UserID = dbo.Deliveries.UserID 

Answer 1

需要两个不同的连接到Users。你还需要学会使用表别名：

SELECT pu.FirstName as purchase_FirstName, pu.LastName as purchase_LastName, 
     du.FirstName as delivery_FirstName, du.LastName as delivery_LastName, 
     po.PurchaseOrderDate, 
     d.ExpectedDeliveryDate, d.ActualDeliveryDate 
FROM dbo.PurchaseOrders po JOIN 
    dbo.Deliveries d 
    ON po.PurchaseOrderID = d.PurchaseOrderID JOIN 
    dbo.Users pu 
    ON p.UserID = pu.UserID JOIN 
    dbo.Users du 
    ON d.UserId = du.UserId; 

你的查询没有返回的，因为这种情况记录：

AND dbo.Users.UserID = dbo.PurchaseOrders.UserID 
    AND dbo.Users.UserID = dbo.Deliveries.UserID 

这显然意味着，dbo.PurchaseOrders.UserID = dbo.Deliveries.UserID。所以，如果这不是真的，那么没有记录符合条件。

Answer 2

当您使用Inner Join时，如果数据不匹配，您将不会得到任何记录。您需要使用LEFT JOIN，并且第二次连接不需要使用用户标识的条件。尝试下面的查询。

SELECT 
dbo.Users.FirstName, 
    dbo.Users.LastName, 
    dbo.PurchaseOrders.PurchaseOrderDate, 
    dbo.Deliveries.ExpectedDeliveryDate, 
    dbo.Deliveries.ActualDeliveryDate 
FROM dbo.PurchaseOrders 
INNER JOIN dbo.Users 
    ON dbo.PurchaseOrders.UserID = dbo.Users.UserID 
LEFT JOIN dbo.Deliveries 
    ON dbo.PurchaseOrders.PurchaseOrderID = dbo.Deliveries.PurchaseOrderID