sql server - check to see if cast is possible - sql

Question

I have the following code to cast nvarchar to integer:
cast(#value as int)
However I have no control of the parameter #value, hence the code might fail. Is there anyway to check if a cast is possible before doing a cast?

Answer 1

Well, in SQL Server 2012 you could use the new TRY_CAST(), but with SQL Server 2008, you should be able to use ISNUMERIC(), and then include handling for values that do not pass that test.

Answer 2

I've recently answered a question about this and using ISNUMERIC to CAST to an INT won't work by itself. Reason being, ISNUMERIC returns true for non integer numbers (1.5) for example.
Here was a recent answer on the subject:
https://stackoverflow.com/a/14692165/1073631
Consider adding an additional check using CHARINDEX with ISNUMERIC, or what I prefer, use a Regular Expression to validate the data.
And here is a Fiddle demonstrating the problem with using ISNUMERIC on it's own. And the Fiddle using a regular expression instead that works.
DECLARE #Test nvarchar(10)
SET #Test = '1.5'
--Works
SELECT CASE WHEN #Test NOT LIKE '%[^0-9]%' THEN CAST(#Test as int) ELSE 0 END
-- Produces Error
SELECT CASE WHEN ISNUMERIC(#Test) = 1 THEN CAST(#Test as int) ELSE 0 END
Good luck.

Answer 3

I generally use the following, it seems to cover all the situations.
SELECT CASE WHEN 1 = ISNUMERIC(#value + '.0') THEN CAST(#value as int) ELSE 0 END
It takes advantage of the fact that "ISNUMERIC" will not allow two periods. The "TRY_CAST" in SQL Server 2012+ is a much better solution though.

Answer 4

The proper test is:
select (case when isnumeric(val) = 1 and val not like '%e%' and val not like '%.%'
then cast(val as int)
end)
The function isnumeric() returns 1 for anything that looks like a float, so you have to be careful.
You can also use what I consider to be a peculiarity of SQL Server. You can cast the floating value 1.23 to an int, but you cannot cast the string value. So, the following also works:
select (case when isnumeric(val) = 1
then cast(cast(val as float) as int)
end)

Answer 5

Maybe we can do something like this:
declare #value as nvarchar(10) = 'A';
begin try
select cast(#value as int);
end try
begin catch
-- do something
end catch

Answer 6

Use a procedure with a TRY CATCH block to suppress errors
i.e.
CREATE PROCEDURE p_try_cast
#type nvarchar(MAX),
#value nvarchar(MAX)
AS
BEGIN
BEGIN TRY
DECLARE #sql varchar(MAX)
DECLARE #out_table TABLE(value varchar(MAX))
SET #sql = 'SELECT CONVERT(varchar(max), CAST(''' + #value + ''' AS ' + #type + '))'
INSERT #out_table
EXECUTE (#sql)
IF EXISTS ( SELECT 1 FROM #out_table WHERE value = #value)
RETURN 1
RETURN 0
END TRY
BEGIN CATCH
RETURN 0
END CATCH
END
GO
Now you can call that with the passed string and desired type and the proc returns 1 for success and 0 for failure
DECLARE #ret int
-- This returns 0 - Fail
EXEC #ret = p_try_cast 'integer', '1.5'
-- This returns 1 - Success
EXEC #ret = p_try_cast 'integer', '1.5'
-- This returns 0 - Fail
EXEC #ret = p_try_cast 'char(4)', 'HELLO'
-- This returns 1 - Success
EXEC #ret = p_try_cast 'char(4)', 'HELL'