集合操作中别名的使用问题
use Northwind
go
select EmployeeID ,CustomerID,OrderID,OrderDate
from ( select 1 as SortCol,CustomerID,EmployeeID,OrderID,OrderDate
from dbo.Orders
where CustomerID = 'ALFKI'
union all
select 2 as SortCol,CustomerID,EmployeeID,OrderID,OrderDate
from dbo.Orders
where EmployeeID = 3 )as U
order by SortCol ,
case when SortCol = 1 then OrderID end,
case when SortCol = 2 then OrderDate end desc;

上面的sql语句使用示例数据库Northwind中的表Orders，当CustomerID
为ALFKI，按OrderID排当EmployeeID = 3按 OrderDate降序排这儿为什么
去掉集合操作中的别名as U会报错？？？？


[最优解释]
from 后面接表名或者子查询名称.你把 as U去掉后,就没有名称了.当然会报错.
[其他解释]
因为 （select 1 as SortCol,CustomerID,EmployeeID,OrderID,OrderDate
from dbo.Orders
where CustomerID = 'ALFKI'
union all
select 2 as SortCol,CustomerID,EmployeeID,OrderID,OrderDate
from dbo.Orders
where EmployeeID = 3 ） 整个 作为了一个名为U的表来使用

简写就是

select EmployeeID ,CustomerID,OrderID,OrderDate from U order by SortCol ,
 case when SortCol = 1 then OrderID end,
 case when SortCol = 2 then OrderDate end desc;

[其他解释]
楼上回答的我知道,现在如果不使用别名U，也就是去掉as U, 就会报错,这个是怎么回事？？？？