Error Handling in T-SQL Scalar Function

Question

This question could easily take multiple paths, so I will hit the more specific path first. While working with SQL Server 2005, I'm trying to create a scalar function that acts as a 'TryCast' from varchar to int. Where I encounter a problem is when I add a TRY block in the function;

CREATE FUNCTION u_TryCastInt
(
   @Value as VARCHAR(MAX)
)
RETURNS Int
AS
BEGIN
   DECLARE @Output AS Int

   BEGIN TRY
      SET @Output = CONVERT(Int, @Value)
   END TRY
   BEGIN CATCH
      SET @Output = 0
   END CATCH

   RETURN @Output
END

Turns out theres all sorts of things wrong with this statement including "Invalid use of side-effecting or time-dependent operator in 'BEGIN TRY' within a function" and "Invalid use of side-effecting or time-dependent operator in 'END TRY' within a function". I can't seem to find any examples of using try statements within a scalar function, which got me thinking, is error handling in a function is possible?

The goal here is to make a robust version of the Convert or Cast functions to allow a SELECT statement carry through depsite conversion errors. For example, take the following;

    CREATE TABLE tblTest
    (
        f1 VARCHAR(50)
    )
    GO

    INSERT INTO tblTest(f1) VALUES('1')
    INSERT INTO tblTest(f1) VALUES('2')
    INSERT INTO tblTest(f1) VALUES('3')
    INSERT INTO tblTest(f1) VALUES('f')
    INSERT INTO tblTest(f1) VALUES('5')
    INSERT INTO tblTest(f1) VALUES('1.1')

    SELECT CONVERT(int,f1) AS f1_num FROM tblTest

    DROP TABLE tblTest

It never reaches point of dropping the table because the execution gets hung on trying to convert 'f' to an integer. I want to be able to do something like this;

SELECT u_TryCastInt(f1) AS f1_num FROM tblTest

fi_num
__________
1
2
3
0
5
0

Any thoughts on this? Is there anything that exists that handles this? Also, I would like to try and expand the conversation to support SQL Server 2000 since Try blocks are not an option in that scenario.

Answer 1

Check if you can convert to int first, check out the IsInteger function here: IsNumeric, IsInt, IsNumber It will work on 2000 and up