##ERROR - How to process / evaluate in TSQL? - sql-server-2008

Question

This update will fail as ID is Identity
But I am getting direct error message.
I am getting none of the PRINT statements.
How do I use ##ERROR?
Using SSMS.
Would you call this a script or a query?
PRINT 'start';
DECLARE #ErrorVal INT;
UPDATE IndenText SET ID = 7;
SELECT #ErrorVal = ##ERROR;
PRINT #ErrorVal;
IF #ErrorVal <> 0
BEGIN
PRINT N'A error caught.';
END
PRINT 'done';
This also does not work from me
PRINT 'start';
DECLARE #ErrorVal INT;
BEGIN TRY
UPDATE IndenText SET ID = 7;
SELECT #ErrorVal = ##ERROR;
PRINT #ErrorVal;
END TRY
BEGIN CATCH
IF ##ERROR <> 0
BEGIN
PRINT N'A error caught.' + ##ERROR;
END
END CATCH
PRINT 'done';
I get
Msg 8102, Level 16, State 1, Line 4
Cannot update identity column 'ID'.
As Martin stated it was a compile error.
Created a constraint that would not be a compile error.
And got the ##ERROR to process.
PRINT 'start';
DECLARE #ErrorVal INT;
DECLARE #newVal INT;
select #newVal = -1;
BEGIN TRY
update Twaste1 set ID = #newVal ;
PRINT 'End Try';
END TRY
BEGIN CATCH
Select #ErrorVal = ##ERROR;
PRINT 'Begin Catch';
IF #ErrorVal <> 0
BEGIN
PRINT CAST(#ErrorVal as varchar(30));
PRINT N'A error caught.';
END
END CATCH
PRINT 'done'
;

Answer 1

For example For Update Use :
UPDATE HumanResources.EmployeePayHistory
SET PayFrequency = 4
WHERE BusinessEntityID = 1;
IF ##ERROR = 547
PRINT N'A check constraint violation occurred.';
GO

Answer 2

I would suggest using TRY... CATCH instead. This will allow you to capture and examine the error thrown.
UPDATE:
I believe Martin already explained why this doesn't work in his comments to the original question, but I'll add a reference to the MSDN TRY... CATCH article:
The following types of errors are not handled by a CATCH block when
they occur at the same level of execution as the TRY…CATCH construct:
Compile errors, such as syntax errors, that prevent a batch from running.
Errors that occur during statement-level recompilation, such as object name resolution errors that occur after compilation because
of deferred name resolution.