Tuesday, January 31, 2017

SQL ISNUMERIC() function

Imagine you have a column of data which is supposed to contain numbers, but you don't know which ones are numbers. Imaging you also live in a world where SQL Server 2012 and above don't exist. Now imagine you stumble across the function ISNUMERIC one day and think to yourself "huzzah! My worries are over!". Now imagine you put that check in to your code, push it to production, and low and behold, it doesn't do what you expect.

This is documented elsewhere as well but ISNUMERIC might not do what you expect. It includes characters which aren't strictly speaking numeric values. For instance, it contains several currency signs. It allows some white space characters, and it allows commas, pluses, minuses and more. Consider the following.

    Idx = num,
    Chr = char(num),
    IsNum = isnumeric(char(num)),
    IsNum_e0 = isnumeric(char(num) + 'e0')
from (select top 255 num = row_number() over (order by (select null))
      from sys.all_objects) a
order by isnumeric(char(num)) desc, num 
You can see all the characters (from the first 255 ascii characters) which it will consider to be "numeric". It's also worth noting that this is not a comprehensive list of characters though, as I'll illustrate in a way which improves the functionality of isnumeric in many cases. the trick is to add 'e0' to the end of the string you're converting. So 123 becomes 123e0. Since this is scientific notation, it's considered a numeric value; and rightfully so. but "$\e0" is decidedly NOT numeric. In the statement above, I included a "e0" suffixed column as well as you can see it cuts out everything that's not a number.

So remember ISNUMERIC is not quite what you think, but if you're expecting a run-of-the-mill integer, float, or decimal value, appending "e0" to the end can get you a long way.

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