Sql Server Uses Data Which Was Filtered Before

I got this table named 'Streets':







Street (varchar)
PLZ (varchar)

'A'
'?'

'B'
'1'



SELECT * FROM (

SELECT * FROM (

SELECT Strasse,PLZ FROM Streets

) SUB WHERE IsNumeric(PLZ) = 1

) SUB WHERE PLZ = 1



I always receive this error:

Conversion failed when converting the nvarchar value '?' to data type int.



How must the statement look like, to execute this successfully?

I also tried to use a view which returns only valid streets, also a function, but it seems sqlserver tests the row with the '?' anyways



PLZ HELP!



PS: If you want to try it, create the table because using the following instead the table works fine:



SELECT 'A' as Street ,'?' as PLZ

UNION

SELECT 'B','1'