Hypothetical scenario: Data has been loaded into a SQL Server table from a CSV file; each column in the table has been defined as VARCHAR(255) to prevent the data load job from failing if one or more values are not of the expected data type. You're job is to copy data from this 'raw' table into one where the columns are strongly typed (i.e. INT, DATETIME, DECIMAL, etc). The requirement is to pull through those rows which are entirely valid, but to ignore those rows where any value in the VARCHAR column cannot be successfully converted to the target data type. Actually, if rows which are only partially valid could be copied across too, with invalid values being set to NULL in the target table, then that would be even better.
The requirement seems simple enough, but how would you do this? It's actually quite tricky to get right. So I'm going to make the scenario even simpler - you only have to support INT data types. Your job is simply to make sure that the supplied VARCHAR can be converted to an INT before you actual attempt to do so.
You first port-of-call might be to use ISNUMERIC to establish whether a source value is numeric. This sounds reasonable enough.
The above statement returns 1 - the value '9876543210' is numeric. But is it an INT? No - it's too large. The documentation for ISNUMERIC says that it returns 1 if the supplied character column can be converted to at least one of the numeric data types. So that's not particularly useful if we're trying to establish whether a value is a valid INT or not.