nested SELECT statements interact in ways that I don't understand - tsql

Question

I thought I understood how I can do a SELECT from the results of another SELECT statement, but there seems to be some sort of blurring of scope that I don't understand. I am using SQL Server 2008R2.
It is easiest to explain with an example.
Create a table with a single nvarchar column - load the table with a single text value and a couple of numbers:
CREATE TABLE #temptable( a nvarchar(30) );
INSERT INTO #temptable( a )
VALUES('apple');
INSERT INTO #temptable( a )
VALUES(1);
INSERT INTO #temptable( a )
VALUES(2);
select * from #temptable;
This will return: apple, 1, 2
Use IsNumeric to get only the rows of the table that can be cast to numeric - this will leave the text value apple behind. This works fine.
select cast(a as int) as NumA
from #temptable
where IsNumeric(a) = 1 ;
This returns: 1, 2
However, if I use that exact same query as an inner select, and try to do a numeric WHERE clause, it fails saying cannot convert nvarchar value 'apple' to data type int. How has it got the value 'apple' back??
select
x.NumA
from
(
select cast(a as int) as NumA
from #temptable
where IsNumeric(a) = 1
) x
where x.NumA > 1
;
Note that the failing query works just fine without the WHERE clause:
select
x.NumA
from
(
select cast(a as int) as NumA
from #temptable
where IsNumeric(a) = 1
) x
;
I find this very surprising. What am I not getting? TIA

Answer 1

If you take a look at the estimated execution plan you'll find that it has optimized the inner query into the outer and combined the WHERE clauses.
Using a CTE to isolate the operations works (in SQL Server 2008 R2):
declare #temptable as table ( a nvarchar(30) );
INSERT INTO #temptable( a )
VALUES ('apple'), ('1'), ('2');
with Numbers as (
select cast(a as int) as NumA
from #temptable
where IsNumeric(a) = 1
)
select * from Numbers

Answer 2

The reason you are getting this is fair and simple. When a query is executed there are some steps that are being followed. This is a parse, algebrize, optimize and compile.
The algebrize part in this case will get all the objects you need for this query. The optimize will use these objects to create a best query plan which will be compiled and executed...
So, when you look into that part you will see it will do a table scan on #temptable. And #temptable is defined as the way you created your table. That you will do some compute on it is a different thing..... The column still has the nvarchar datatype..
To know how this works you have to know how to read a query. First all the objects are retrieved (from table, inner join table), then the predicates (where, on), then the grouping and such, then the select of the columns (with the cast) and then the orderby.
So with that in mind, when you have a combination of selects, the optimizer will still process it that way.. since your select is subordinate to the from and join parts of your query, it will be a reason for getting this error.
I hope i made it a little clear?

Answer 3

The optimizer is free to move expressions in the query plan in order to produce the most cost efficient plan for retrieving the data (the evaluation order of the predicates is not guaranteed). I think using the case expression like bellow produces a NULL in absence of the ELSE clause and thus takes the APPLE out
select a from #temptable where case when isnumeric(a) = 1 then a end > 1