error in multi batch transactional sql script - sql-server

Question

BEGIN TRAN
SET XACT_ABORT ON
GO
BEGIN TRY
IF OBJECT_ID('dbo.Offer_GetByStudyId', 'p') IS NULL
EXEC ('CREATE PROCEDURE Offer_GetByStudyId as select 1')
END TRY
BEGIN CATCH
THROW;
END CATCH
GO
IF ##error <> 0 and ##trancount > 0 ROLLBACK
IF ##trancount = 0 BEGIN SET NOCOUNT ON; SET NOEXEC ON; END
GO
BEGIN TRY
ALTER PROCEDURE dbo.Offer_GetByStudyId
#StudyId NVARCHAR(MAX) = NULL
AS
BEGIN
DECLARE #Conditions NVARCHAR(MAX) = '';
IF #StudyId IS NOT NULL
BEGIN
SET #Conditions = #Conditions + ' AND o.StudyId = ' + cast(#StudyId as varchar(10))
END
DECLARE #sql NVARCHAR(MAX) = 'SELECT
o.StudyId as StudyId,
o.SampleId as SampleId,
o.Status as Status,
o.Title as Title,
o.Topic as Topic,
o.Description as Description,
o.TestOffer as TestOffer,
T.CPI as CPI
FROM Offers o
LEFT JOIN [dbo].[Terms] T ON (o.[Id] = T.[OfferId]) AND T.Active = 1
WHERE 1 = 1' + #Conditions
EXEC(#sql)
END
END TRY
BEGIN CATCH
THROW;
END CATCH
This is my SQL script, I'm trying to have a multi batch script run as a single transaction, so if one statement fails, all of it will be rolled back. But I here keep getting this error:
Incorrect syntax near BEGIN. expecting EXTERNAL
The begin they are talking about is the one after:
#StudyId NVARCHAR(MAX) = NULL

Answer 1

You need to remove your "GO" statements. These are not part of the TSQL language, they are just statements to tell SSMS/SQLCMD that the batch above should be executed. I'm not clear on the behavior of a transaction split across several GO statements. I would start my removing the "GO"s.
https://msdn.microsoft.com/en-us/library/ms188037.aspx

Answer 2

If you take out the statement ALTER PROCEDURE dbo.Offer_GetByStudyId (Line 22 - 47) from TRY CATCH it will work. i.e delete TRY CATCH (Line 20 and Line 48 - 51)

Answer 3

I've realised I've buried the answer in the below, so I'll bring it up to the top to make it clearer: you cannot wrap any flow control statements around an attempt to create or alter a procedure
A simpler example demonstrates the problem:
create procedure ABC
as
go
select * from sys.objects
alter procedure ABC as
Which produces the message:
Msg 111, Level 15, State 1, Procedure ABC, Line 2
'CREATE/ALTER PROCEDURE' must be the first statement in a query batch.
This is explicitly documented for CREATE PROCEDURE but for some reason isn't for ALTER PROCEDURE.
The CREATE PROCEDURE statement cannot be combined with other Transact-SQL statements in a single batch.
So the upshot is, you cannot wrap any flow control statements around an attempt to create or alter a procedure.
The reason is actually pretty simple - BEGIN and END aren't required around the body of the stored procedure - and a stored procedure can actually contain multiple "top-level" BEGIN/END pairs:
create procedure ABC as
begin
select * from sys.objects
end
begin
select * from sys.columns
end
is fine - so the only way that SQL Server knows the extent of a stored procedure, when it's being defined, is "from the CREATE/ALTER PROCEDURE until the end of the batch."