When is a number not a number?
Bob Dorr has posted about conversion issues with isnumeric
This reminds me of a difficult "bug" in an application whereby a lookup actually entered a numeric value into a table from a text list. For the user the list was too long - why some developers think a drop down list of over a thousand entries is a "good idea" I never know - anyway the users had discovered that you could bypass the drop down to get an unknown by entering a full stop ( decimal point ) Now if you type
Select isnumeric('.'); this will return true/1 , however select convert(int,'.'); will fail.
Sadly for the application in question the data entry didn't fail it was a process which occurred somewhat later - it took some considerable time to sort it out. Personally I wouldn't allow users to enter commas and full stops but it's worth noting that the logic behind a function doesn't always follow what we might consider "commonsense decisions" as amany a bug will prove.