Difference between 'With A as' and 'as A' in SQL-CodePudding

I have a Database named Students, with the attribute 'Grade' to store the grades of students. I wrote 2 following spinnets of code and expected that I would have the same result:

SELECT * FROM (SELECT * FROM dbo.Students) AS A
WHERE A.Grade = (SELECT MAX(Grade) FROM A)

WITH A AS (SELECT * FROM Students)
SELECT * FROM A 
WHERE A.Grade = (SELECT MAX(Grade) FROM A)

But actually, when I executed the first code, I got an error like "Invalid object name 'A'." I cannot understand why I got that error because I think the logic of the two codes is the same. Can anyone explain this to me, please!!

Thank you very much!!

CodePudding user response：

You name a subquery here:

...(SELECT * FROM dbo.Students) AS A -- here

, but use this alias (A) inside same query here:

...(SELECT MAX(Grade) FROM A) -- here

It's not allowed.
When you use a CTE (WITH A AS...), you claim A at first and then use it properly.