转换数字与isnumeric

Question

这有效并返回1.

<cfquery name="test">
DECLARE ID varchar(50) = '8D4'

SELECT ISNUMERIC("8D4") AS isnum
</cfquery>

这失败

<cfquery name="test">
DECLARE ID varchar(50) = '8D4'

CASE WHEN ISNUMERIC(@ID) = 1 THEN CONVERT(bigint, @ID) ELSE NULL END AS Myid
</cfquery>

我应该进行什么样的测试才能确信CONVERT会起作用？