SQL: multiple statements within a BEGIN and END - sql

Question

I'm trying to add a new column to an existing table which will be populated by a unique Guid value. I'm trying the following code:
IF NOT EXISTS(select * from sys.columns
where Name = N'Product_GUID' and Object_ID = Object_ID(N'dbo.Product'))
BEGIN
PRINT 'Creating new GUID column in dbo.Product table'
ALTER TABLE dbo.Product
ADD Product_GUID uniqueidentifier NULL
UPDATE dbo.Product
SET Product_Guid=NEWID()
ALTER TABLE dbo.Product
ALTER COLUMN Product_Guid uniqueidentifier NOT NULL
END
This won't work becasue the second statement doesn't recognise the new column name. I can't put GO or ; at the end of each statement though, presumably because I'm in the middle of the BEGIN/END block.
What's the best way of solving this dilemma?

Answer 1

The statement doing the update must be compiled after the column is added.
The way this is usually done is wrapping the statements in an EXEC:
EXEC(' UPDATE dbo.Product
SET Product_Guid = NEWID()
ALTER TABLE dbo.Product
ALTER COLUMN Product_Guid uniqueidentifier NOT NULL
')

Answer 2

It seems like you want to set the default value and have the column be not null. You'll get the same effect if you just set the default value to NEWID()
IF NOT EXISTS(select * from sys.columns
where Name = N'Product_GUID' and Object_ID = Object_ID(N'dbo.Product'))
BEGIN
PRINT 'Creating new GUID column in dbo.Product table'
ALTER TABLE dbo.Product
ADD Product_GUID uniqueidentifier NOT NULL DEFAULT NEWID()
END
If you need to remove the constraint after, you can create the DEFAULT constraint after you define the column in the alter statement and then drop the named constraint right after. If you don't name the constraint you'll have to get the name from sys.objects and then do dynamic sql to remove it.
IF NOT EXISTS(select * from sys.columns
where Name = N'Product_GUID' and Object_ID = Object_ID(N'dbo.Product'))
BEGIN
PRINT 'Creating new GUID column in dbo.Product table'
ALTER TABLE dbo.Product
ADD Product_GUID uniqueidentifier NOT NULL,
CONSTRAINT Default_Product_GUID DEFAULT NEWID() FOR Product_GUID;
ALTER TABLE dbo.Product DROP CONSTRAINT Default_Product_GUID
END

Answer 3

You could just update the table afterwards and then alter it in another code block, a little something like this:
IF NOT EXISTS(select * from sys.columns
where Name = N'Product_GUID' and Object_ID = Object_ID(N'dbo.Product'))
BEGIN
PRINT 'Creating new GUID column in dbo.Product table'
ALTER TABLE dbo.Product
ADD Product_GUID uniqueidentifier NULL
END
GO
UPDATE dbo.Product
SET Product_Guid=NEWID()
Where Product_Guid is null
if ##ROWCOUNT <> 0
Begin
ALTER TABLE dbo.Product
ALTER COLUMN Product_Guid uniqueidentifier NOT NULL
End