Error on select when using value converted from varchar to integer (with isnumeric in where)

Question

Hi,
Why is it, that when I convert a varchar into integer and filter it to numeric only that I get the Error 'Conversion failed when converting the nvarchar value '...' to data type int.'
Here is a short SQL witch shows the Error:
create table tTest(ID int, sText nvarchar(15))
insert into tTest
select 1, null
insert into tTest
select 2, 'q'
go
select distinct ID, sText, convert(int,sText) AS ID2 from tTest where isnumeric(sText) = 1
go
SELECT DISTINCT TOP 100 PERCENT
ID
FROM
(select distinct ID, sText, cast(sText as int) AS ID2 from tTest where isnumeric(sText) = 1) v1
WHERE
id2 = 1
go
drop table tTest
As a workaround I replaced the case with 'cast(case when isnumeric(sText) = 1 then sText else null end as int)', but this rater unexpected.
It works fine in a short query, but if I create a view with the SQL from V1 then the error is unexpected, since I only know that if I view the source of the view

Answer 1

>'cast(case when isnumeric(sText) = 1 then sText else null
ISNUMERIC is not 100% accurate.
BOL: "ISNUMERIC returns 1 for some characters that are not numbers, such as plus (+), minus (-), and valid currency symbols such as the dollar sign ($). For a complete list of currency symbols, see money and smallmoney (Transact-SQL)."
LINK: http://technet.microsoft.com/en-us/library/ms186272.aspx
ISNUMERIC examples:
http://sqlusa.com/bestpractices/isnumeric/
Kalman Toth Database & OLAP Architect
Free T-SQL Scripts
New Book / Kindle: Exam 70-461 Bootcamp: Querying Microsoft SQL Server 2012