子查询中的SQL Server ORDER BY子句

Question

I am facing a strange error in SQL Server and I want some explanation of it.

When I write ORDER BY in a subquery, for instance

SELECT a FROM (SELECT * FROM A ORDER BY a) T

it throws the following error

The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP or FOR XML is also specified.

But when I use TOP in subquery it works normally

 SELECT a 
 FROM
    (SELECT TOP 1000000 * FROM A ORDER BY a) T

So, does it mean that I can select top row count of A, instead of

SELECT a FROM (SELECT * FROM A ORDER BY a) T

In that case. what is the reason of error?

Answer 1

There is no much sense to sort the subquery and after that select something from it - it is not guaranteed that top-level select will be ordered, so - there is no sense to order the inner query

But if you order inner query with TOP statement - it also not guaranteed that top level select will be ordered in such a way, but it will contain only top X rows from the inner query - that is already makes sense.