How to query by digits in a char column

PROBLEM:
You have a char column that can has (logically) only two types of data for each record: digits or text. You want to make a query using comparison operators (Greater than, Less than..) to fetch only records contains digits.
You can use the isnumeric() function, but the following problems occur:

SELECT ISNUMERIC('1212') --> 1: exactly

SELECT ISNUMERIC('12C2') --> 0: exactly

SELECT ISNUMERIC('10E0') --> 1: wrong

SELECT ISNUMERIC('1,100') --> 1: wrong

SELECT ISNUMERIC('+') --> 1: wrong

SELECT ISNUMERIC('-') --> 1: wrong

SOLUTION:
As mentioned above, values like minus sign '-' and the plus sign '+' by themselves will satisfy isnumeric() but cannot be used for the "is digit" query. For this, it is better to choose the wildcard approach

SELECT

    digit_idnum

FROM

(

    select case

            IsNull((select 1

                    where idnum not like '%[^0-9]%'),0)

            when 1 then CAST(idnum as int)

            end as digit_idnum

    from my_table

) AS t

WHERE

digit_idnum >=  200 and

digit_idnum <=  700

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