使用Top with Union全部

Question

在SQL Server 2014中，我使用Northwind作为示例数据库，并使用Orders表。

选择第一行：

select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders 

输出：

orderID CustomerID EmployeeID OrderDate RequiredDate 
10248 VINET 5 1996-07-04 00:00:00.000 1996-08-01 00:00:00.000 

用于选择的最后一行：

select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders order by orderID desc 

输出：

orderID CustomerID EmployeeID OrderDate RequiredDate 
11077 RATTC 1 1998-05-06 00:00:00.000 1998-06-03 00:00:00.000 

让合并第一和最后一行通过使用Union ALL：

select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders 
union all 
select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders order by orderID desc 

输出：

orderID CustomerID EmployeeID OrderDate RequiredDate 
10248 VINET 5 1996-07-04 00:00:00.000 1996-08-01 00:00:00.000 
10248 VINET 5 1996-07-04 00:00:00.000 1996-08-01 00:00:00.000 

那么，为什么最后的结果不显示数据符合市场预期。

我知道得到第一和最后一排，用下一个： -

select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders 
union all 
Select * from 
(
select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders order by orderID desc) a 

输出： -

orderID CustomerID EmployeeID OrderDate RequiredDate 
10248 VINET 5 1996-07-04 00:00:00.000 1996-08-01 00:00:00.000 
11077 RATTC 1 1998-05-06 00:00:00.000 1998-06-03 00:00:00.000 

但我仍然感到困惑，为什么我应该，但第二选择成派生表！

Answer 1

这

select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders 
union all 
select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders order by orderID desc 

实际工作为：

(select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders 
union all 
select top 1 orderID, CustomerID,EmployeeID,OrderDate,RequiredDate 
from orders) order by orderID desc 

，因为是在最后的联合评估，除非它被包裹的子查询中的顺序。

您已经知道解决方法，所以我没有提及它。