MS SQL-Server - How to select numeric rows from a varchar column

Luckily T-SQL has an ISNUMERIC(value) function:

select ISNUMERIC('PostalCode123') -- Returns 0
select ISNUMERIC('123') -- Returns 1

Therefore:

select *
from Customer
where ISNUMERIC(PostalCode) = 1

...returns only the rows where the varchar column PostalCode is numeric.