Try_Convert pour SQL Server 2008 R2

Lors de l'utilisation de XML dans SQL Server, vous pouvez essayer cast vers un type de données et de recevoir des valeurs null où la conversion échoue.

declare @T table
(
  Col varchar(50)
)

insert into @T values
('1'),
('1.1'),
('1,1'),
('1a')

select cast('' as xml).value('sql:column("Col") cast as xs:decimal ?', 
                             'decimal(28,10)') as Col
from @T

Résultat:

Col
-------------
1.0000000000
1.1000000000
NULL
NULL

Vous pouvez écrire votre propre parser en C# et de l'utilisation SQLCLR, en utilisant par exemple Décimal.Parse(). Ne pas atempt à utiliser ISNUMERIC, est notoirement incorrect (renvoie VRAI pour les chaînes qui ne parviennent pas à jeter).

J'ai écrit une utile fonction scalaire pour simuler la TRY_CAST fonction de SQL SERVER 2012 SQL Server 2008.

dbo.TRY_CAST(Expression, Data_Type, ReturnValueIfErrorCast)

Les deux principales différences avec TRY_CAST Fonction fo SQL Server 2012 sont que vous devez passer 3 paramètres et vous
doit en outre effectuer explicite CONVERTIR ou en FONTE sur le terrain.
Cependant, il est toujours très utile car il vous permet de retourner un
valeur par défaut si la conversion n'est pas effectuée correctement.

CODE DE LA FONCTION:

DECLARE @strSQL NVARCHAR(1000)
IF NOT EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[dbo].[TRY_CAST]'))
BEGIN
SET @strSQL = 'CREATE FUNCTION [dbo].[TRY_CAST] () RETURNS INT AS BEGIN RETURN 0 END'
EXEC sys.sp_executesql @strSQL
END
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
/*
------------------------------------------------------------------------------------------------------------------------
Description:    
Syntax 
---------------
dbo.TRY_CAST(Expression, Data_Type, ReturnValueIfErrorCast)
+---------------------------+-----------------------+
|   Expression              |   VARCHAR(8000)       |
+---------------------------+-----------------------+
|   Data_Type               |   VARCHAR(8000)       |
+---------------------------+-----------------------+
|   ReturnValueIfErrorCast  |   SQL_VARIANT = NULL  |
+---------------------------+-----------------------+
Arguments
---------------
expression
The value to be cast. Any valid expression.
Data_Type
The data type into which to cast expression.
ReturnValueIfErrorCast
Value returned if cast fails or is not supported. Required. Set the DEFAULT value by default.
Return Type
----------------
Returns value cast to SQL_VARIANT type if the cast succeeds; otherwise, returns null if the parameter @pReturnValueIfErrorCast is set to DEFAULT, 
or that the user indicates.
Remarks
----------------
dbo.TRY_CAST function simulates the TRY_CAST function reserved of SQL SERVER 2012 for using in SQL SERVER 2008. 
dbo.TRY_CAST function takes the value passed to it and tries to convert it to the specified Data_Type. 
If the cast succeeds, dbo.TRY_CAST returns the value as SQL_VARIANT type; if the cast doesn´t succees, null is returned if the parameter @pReturnValueIfErrorCast is set to DEFAULT. 
If the Data_Type is unsupported will return @pReturnValueIfErrorCast.
dbo.TRY_CAST function requires user make an explicit CAST or CONVERT in ANY statements.
This version of dbo.TRY_CAST only supports CAST for INT, DATE, NUMERIC and BIT types.
Examples
====================================================================================================
--A. Test TRY_CAST function returns null
SELECT   
CASE WHEN dbo.TRY_CAST('6666666166666212', 'INT', DEFAULT) IS NULL   
THEN 'Cast failed'  
ELSE 'Cast succeeded'  
END AS Result; 
GO
--B. Error Cast With User Value
SELECT   
dbo.TRY_CAST('2147483648', 'INT', DEFAULT) AS [Error Cast With DEFAULT],
dbo.TRY_CAST('2147483648', 'INT', -1) AS [Error Cast With User Value],
dbo.TRY_CAST('2147483648', 'INT', NULL) AS [Error Cast With User NULL Value]; 
GO 
--C. Additional CAST or CONVERT required in any assignment statement
DECLARE @IntegerVariable AS INT
SET @IntegerVariable = CAST(dbo.TRY_CAST(123, 'INT', DEFAULT) AS INT)
SELECT @IntegerVariable
GO 
IF OBJECT_ID('tempdb..#temp') IS NOT NULL
DROP TABLE #temp
CREATE TABLE #temp (
Id INT IDENTITY
, FieldNumeric NUMERIC(3, 1)
)
INSERT INTO dbo.#temp (FieldNumeric)
SELECT CAST(dbo.TRY_CAST(12.3, 'NUMERIC(3,1)', 0) AS NUMERIC(3, 1));--Need explicit CAST on INSERT statements
SELECT *
FROM #temp
DROP TABLE #temp
GO 
--D. Supports CAST for INT, DATE, NUMERIC and BIT types.
SELECT dbo.TRY_CAST(2147483648, 'INT', 0) AS [Cast failed]
, dbo.TRY_CAST(2147483647, 'INT', 0) AS [Cast succeeded]
, SQL_VARIANT_PROPERTY(dbo.TRY_CAST(212, 'INT', 0), 'BaseType') AS [BaseType];
SELECT dbo.TRY_CAST('AAAA0101', 'DATE', DEFAULT) AS [Cast failed]
, dbo.TRY_CAST('20160101', 'DATE', DEFAULT) AS [Cast succeeded]
, SQL_VARIANT_PROPERTY(dbo.TRY_CAST('2016-01-01', 'DATE', DEFAULT), 'BaseType') AS [BaseType];
SELECT dbo.TRY_CAST(1.23, 'NUMERIC(3,1)', DEFAULT) AS [Cast failed]
, dbo.TRY_CAST(12.3, 'NUMERIC(3,1)', DEFAULT) AS [Cast succeeded]
, SQL_VARIANT_PROPERTY(dbo.TRY_CAST(12.3, 'NUMERIC(3,1)', DEFAULT), 'BaseType') AS [BaseType];
SELECT dbo.TRY_CAST('A', 'BIT', DEFAULT) AS [Cast failed]
, dbo.TRY_CAST(1, 'BIT', DEFAULT) AS [Cast succeeded]
, SQL_VARIANT_PROPERTY(dbo.TRY_CAST('123', 'BIT', DEFAULT), 'BaseType') AS [BaseType];
GO 
--E. B. TRY_CAST return NULL on unsupported data_types
SELECT dbo.TRY_CAST(4, 'xml', DEFAULT) AS [unsupported];  
GO  
====================================================================================================
------------------------------------------------------------------------------------------------------------------------
Responsible:    Javier Pardo 
Date:           diciembre 29/2016
WB tests:       Javier Pardo 
------------------------------------------------------------------------------------------------------------------------
Update by:      Javier Eduardo Pardo Moreno 
Date:           febrero 16/2017
Id update:      JEPM20170216
Description:    Fix  ISNUMERIC function makes it unreliable. SELECT dbo.TRY_CAST('+', 'INT', 0) will yield Msg 8114, 
Level 16, State 5, Line 16 Error converting data type varchar to float.
ISNUMERIC() function treats few more characters as numeric, like: – (minus), + (plus), $ (dollar), \ (back slash), (.)dot and (,)comma
Collaborator aperiooculus (http://stackoverflow.com/users/3083382/aperiooculus )
Fix dbo.TRY_CAST('2013/09/20', 'datetime', DEFAULT) for supporting DATETIME format
WB tests:       Javier Pardo
------------------------------------------------------------------------------------------------------------------------
*/
ALTER FUNCTION dbo.TRY_CAST
(
@pExpression AS VARCHAR(8000),
@pData_Type AS VARCHAR(8000),
@pReturnValueIfErrorCast AS SQL_VARIANT = NULL
)
RETURNS SQL_VARIANT
AS
BEGIN
--------------------------------------------------------------------------------
--  INT 
--------------------------------------------------------------------------------
IF @pData_Type = 'INT'
BEGIN
IF ISNUMERIC(@pExpression) = 1 AND @pExpression NOT IN ('-','+','$','.',',','\')    --JEPM20170216
BEGIN
DECLARE @pExpressionINT AS FLOAT = CAST(@pExpression AS FLOAT)
IF @pExpressionINT BETWEEN - 2147483648.0 AND 2147483647.0
BEGIN
RETURN CAST(@pExpressionINT as INT)
END
ELSE
BEGIN
RETURN @pReturnValueIfErrorCast
END --FIN IF @pExpressionINT BETWEEN - 2147483648.0 AND 2147483647.0
END
ELSE
BEGIN
RETURN @pReturnValueIfErrorCast
END -- FIN IF ISNUMERIC(@pExpression) = 1
END -- FIN IF @pData_Type = 'INT'
--------------------------------------------------------------------------------
--  DATE    
--------------------------------------------------------------------------------
IF @pData_Type IN ('DATE','DATETIME')
BEGIN
IF ISDATE(@pExpression) = 1
BEGIN
DECLARE @pExpressionDATE AS DATETIME = cast(@pExpression AS DATETIME)
IF @pData_Type = 'DATE'
BEGIN
RETURN cast(@pExpressionDATE as DATE)
END
IF @pData_Type = 'DATETIME'
BEGIN
RETURN cast(@pExpressionDATE as DATETIME)
END
END
ELSE 
BEGIN
DECLARE @pExpressionDATEReplaced AS VARCHAR(50) = REPLACE(REPLACE(REPLACE(@pExpression,'\',''),'/',''),'-','')
IF ISDATE(@pExpressionDATEReplaced) = 1
BEGIN
IF @pData_Type = 'DATE'
BEGIN
RETURN cast(@pExpressionDATEReplaced as DATE)
END
IF @pData_Type = 'DATETIME'
BEGIN
RETURN cast(@pExpressionDATEReplaced as DATETIME)
END
END
ELSE
BEGIN
RETURN @pReturnValueIfErrorCast
END
END --FIN IF ISDATE(@pExpression) = 1
END --FIN IF @pData_Type = 'DATE'
--------------------------------------------------------------------------------
--  NUMERIC 
--------------------------------------------------------------------------------
IF @pData_Type LIKE 'NUMERIC%'
BEGIN
IF ISNUMERIC(@pExpression) = 1
BEGIN
DECLARE @TotalDigitsOfType AS INT = SUBSTRING(@pData_Type,CHARINDEX('(',@pData_Type)+1,  CHARINDEX(',',@pData_Type) - CHARINDEX('(',@pData_Type) - 1)
, @TotalDecimalsOfType AS INT = SUBSTRING(@pData_Type,CHARINDEX(',',@pData_Type)+1,  CHARINDEX(')',@pData_Type) - CHARINDEX(',',@pData_Type) - 1)
, @TotalDigitsOfValue AS INT 
, @TotalDecimalsOfValue AS INT 
, @TotalWholeDigitsOfType AS INT 
, @TotalWholeDigitsOfValue AS INT 
SET @pExpression = REPLACE(@pExpression, ',','.')
SET @TotalDigitsOfValue = LEN(REPLACE(@pExpression, '.',''))
SET @TotalDecimalsOfValue = CASE Charindex('.', @pExpression)
WHEN 0
THEN 0
ELSE Len(Cast(Cast(Reverse(CONVERT(VARCHAR(50), @pExpression, 128)) AS FLOAT) AS BIGINT))
END 
SET @TotalWholeDigitsOfType = @TotalDigitsOfType - @TotalDecimalsOfType
SET @TotalWholeDigitsOfValue = @TotalDigitsOfValue - @TotalDecimalsOfValue
-- The total digits can not be greater than the p part of NUMERIC (p, s)
-- The total of decimals can not be greater than the part s of NUMERIC (p, s)
-- The total digits of the whole part can not be greater than the subtraction between p and s
IF (@TotalDigitsOfValue <= @TotalDigitsOfType) AND (@TotalDecimalsOfValue <= @TotalDecimalsOfType) AND (@TotalWholeDigitsOfValue <= @TotalWholeDigitsOfType)
BEGIN
DECLARE @pExpressionNUMERIC AS FLOAT
SET @pExpressionNUMERIC = CAST (ROUND(@pExpression, @TotalDecimalsOfValue) AS FLOAT) 
RETURN @pExpressionNUMERIC --Returns type FLOAT
END 
else
BEGIN
RETURN @pReturnValueIfErrorCast
END-- FIN IF (@TotalDigitisOfValue <= @TotalDigits) AND (@TotalDecimalsOfValue <= @TotalDecimals) 
END
ELSE 
BEGIN
RETURN @pReturnValueIfErrorCast
END --FIN IF ISNUMERIC(@pExpression) = 1
END --IF @pData_Type LIKE 'NUMERIC%'
--------------------------------------------------------------------------------
--  BIT 
--------------------------------------------------------------------------------
IF @pData_Type LIKE 'BIT'
BEGIN
IF ISNUMERIC(@pExpression) = 1
BEGIN
RETURN CAST(@pExpression AS BIT) 
END
ELSE 
BEGIN
RETURN @pReturnValueIfErrorCast
END --FIN IF ISNUMERIC(@pExpression) = 1
END --IF @pData_Type LIKE 'BIT'
--------------------------------------------------------------------------------
--  FLOAT   
--------------------------------------------------------------------------------
IF @pData_Type LIKE 'FLOAT'
BEGIN
IF ISNUMERIC(REPLACE(REPLACE(@pExpression, CHAR(13), ''), CHAR(10), '')) = 1
BEGIN
RETURN CAST(@pExpression AS FLOAT) 
END
ELSE 
BEGIN
IF REPLACE(@pExpression, CHAR(13), '') = '' --Only white spaces are replaced, not new lines
BEGIN
RETURN 0
END
ELSE 
BEGIN
RETURN @pReturnValueIfErrorCast
END --IF REPLACE(@pExpression, CHAR(13), '') = '' 
END --FIN IF ISNUMERIC(@pExpression) = 1
END --IF @pData_Type LIKE 'FLOAT'
--------------------------------------------------------------------------------
--  Any other unsupported data type will return NULL or the value assigned by the user to @pReturnValueIfErrorCast  
--------------------------------------------------------------------------------
RETURN @pReturnValueIfErrorCast
END

Pour l'instant uniquement en charge les types de données INT, DATE, DATETIME, NUMÉRIQUE, BITS et FLOAT. Vous pouvez trouver la dernière versión de ce code dans le lien suivant ci-dessous et nous aider les uns les autres afin de l'améliorer. TRY_CAST Fonction pour SQL Server 2008 https://gist.github.com/jotapardo/800881eba8c5072eb8d99ce6eb74c8bb

CASE WHEN ISNUMERIC(yourColumn) = 1 
THEN CAST(yourColumn AS DECIMAL(28,10)) 
ELSE NULL --or Zero
END AS yourColumName

2019 et toujours en utilisant un 2008 serveur, donc j'ai eu la même question. Le ci-dessus a fonctionné pour moi.