CTE returning error - sql-server-2008

Question

I wrote a CTE to remove non numeric values from a data set, then get a count of numeric values within a range.
WITH dtr
AS ( SELECT resultlevel r
FROM dbo.Result
WHERE DrugID = 'AMP'
AND ISNUMERIC(ResultLevel) = 1
AND AuditStamp > '1/1/2016'
AND DeleteFlag = 0
)
SELECT COUNT(*)
FROM dtr
WHERE CONVERT(INT, r) BETWEEN 50 AND 75
This returns an error in SMS
Msg 245, Level 16, State 1, Line 2
Conversion failed when converting the varchar value 'PND ' to data type int.
This error is completely possible without the 'dtr' query in the CTE.
When I rewrite this, instead of a CTR, but a TEMP table, it works.
SELECT resultlevel r
INTO #d
FROM dbo.Result
WHERE DrugID = 'AMP'
AND ISNUMERIC(ResultLevel) = 1
AND AuditStamp > '1/1/2016'
AND DeleteFlag = 0
SELECT COUNT(*)
FROM #d
WHERE CONVERT(INT, r) BETWEEN 50 AND 75
So my questions is why?? I have always thought a CTE was like creating a TEMP table.
TEST DATA
if object_id('tempdb..#temp') is not null drop table #temp
create table #temp (result char(5))
insert into #temp (result) values
('1'),('A'),('>2'),('PEN ') ,('#3'),('-2'),('-33')
;with isnum AS (
SELECT result
FROM #temp
WHERE ISNUMERIC(result) = 1)
--Selecting from the CTE yields 1, -2, and -33 all of which can be converted to INT
--Running the query with the where clause causes the conversion error
SELECT
result,
ISNUMERIC(result)
FROM isnum
--WHERE CONVERT(INT,result) > 1

Answer 1

In SQL Server there is Logical Processing Order of the SELECT statement, which determines when the objects defined in one step are made available to the clauses in subsequent steps:
FROM
ON
JOIN
WHERE
GROUP BY
WITH CUBE or WITH ROLLUP
HAVING
SELECT
DISTINCT
ORDER BY
TOP
This is how your query is going to be proccesed and your query looks perfectly fine. But sometimes, the SQL Server decides not to follow this order in order to optimize your query.
In your case, the SQL Server might be simplyfing/transforming your query into another and performing the convert function, before applying the where isnumeric filtering.
If we made your query a little more complex (but still giving the same results), the SQL Server is executing the code correctly this time:
;with isnum AS (
SELECT result
FROM #temp
WHERE ISNUMERIC(result) = 1
GROUP BY result
HAVING MAX(result) = result
)
SELECT
result,
ISNUMERIC(result)
FROM isnum
WHERE CONVERT(INT,result) > 1;
In your case (and this is what I am doing in such situations when different types are stored in one column), you can simply use TRY_CONVERT function:
;with isnum AS (
SELECT result
FROM #temp
WHERE ISNUMERIC(result) = 1)
SELECT
result,
ISNUMERIC(result)
FROM isnum
WHERE TRY_CONVERT(INT, result) > 1