SQL Server 2014: How to cast a string value to int only if the value can be casted? - sql

Question

I try to cast a string value in a field of a table to int, but only in the case that the value stores a value that can be casted to int (in the other case the original value must be returned). Example:
DECLARE #ErrorCode nvarchar(1024)
SET #ErrorCode = 'a10'
SELECT IIF(TRY_CAST(#ErrorCode AS int) IS NULL, #ErrorCode, CAST(#ErrorCode AS int))
I've seen similar codes on StackOverflow.
My problem is that the SQL Server (2014) does not seem to short-circuit, and executes the cast always, even if TRY_CAST(#ErrorCode AS int) results in NULL. The result of the code above is the error "Conversion failed when converting the nvarchar value 'a10' to data type int."
See this sample on rextester.com
I also tried other variants with the same result:
SELECT CASE WHEN TRY_CAST(#ErrorCode AS int) IS NULL THEN #ErrorCode ELSE (SELECT CAST(#ErrorCode AS int)) END
SELECT CASE TRY_CAST(#ErrorCode AS int) WHEN 1 THEN CAST(#ErrorCode AS int) ELSE #ErrorCode END
How can I achieve my goal (avoid the cast in case the value in #ErrorCode cannot be casted)?

Answer 1

The simple solution would be to use COALSECE:
DECLARE #ErrorCode nvarchar(1024)
SET #ErrorCode = 'a10'
SELECT COALSECE(CAST(TRY_CAST(#ErrorCode AS int) as nvarchar(1024)), #ErrorCode)
However, I don't see the point of casting to int and then back to nvarchar.

Answer 2

To validate NULL you should IS NULL
DECLARE #ErrorCode NVARCHAR(1024)
SET #ErrorCode = 'a10'
SELECT IIF(TRY_CAST(#ErrorCode AS int) IS NULL, #ErrorCode, CAST(CAST(#ErrorCode AS int) AS VARCHAR(50)))
You need to convert the INT again to VARCHAR again to avoid the implicit conversion. IIF returns the data type with the highest precedence from the types in true_value and false_value.

Answer 3

The problem I see is trying to hold 2 different types in a single column. I'm not sure how you plan to use this information, but you could split the values in to numeric and text columns based on the type. You can do an ISNUMERIC() check and CAST to INT if it's true, otherwise leave it as text, like so:
CREATE TABLE #ErrorCodes ( ErrorCode NVARCHAR(10) )
INSERT INTO #ErrorCodes
( ErrorCode )
VALUES ( '123' ),
( 'a10' ),
( 'bbb' ),
( '456' )
SELECT ErrorCode AS OriginalVal ,
CASE WHEN ISNUMERIC(ErrorCode) = 1 THEN CAST(ErrorCode AS INT)
ELSE NULL
END AS NumericVal ,
CASE WHEN ISNUMERIC(ErrorCode) = 0 THEN ErrorCode
ELSE NULL
END AS NonNumericVal
FROM #ErrorCodes
DROP TABLE #ErrorCodes
Produces:
OriginalVal NumericVal NonNumericVal
=====================================
123 123 NULL
a10 NULL a10
bbb NULL bbb
456 456 NULL