SQL Server 2005从表1和表2选择数据，但如果表2的column1值为空，则从表3选择数

您可以在演示查询中编写case statement或编写ISNULL()如下所示的函数。

SELECT 
    Orders.OrderID, 
    Case when Customers1.CustomerName is null then Customers2.CustomerName else Customers1.CustomerName
        end as CustomerName, --way 1
    ISNULL(Customers1.CustomerName, Customers2.CustomerName) as Customer, --way 2
    Orders.OrderDate
FROM Orders
INNER JOIN Customers1 ON Orders.CustomerID = Customers1.CustomerID
INNER JOIN Customers2 ON Orders.CustomerID = Customers2.CustomerID
-- where your condition here
-- order by your column name

您还可以检查表中是否有数据，并使用if exists如下所示相应地联接表。

if exists(select 1 from tablename where columnname = <your values>)