What is the correct way of using ##IDENTITY - sql-server

Question

Question regarding the ##IDENTITY, I have 4 different tables:
Customer [Id]
Person [Id, fname, lname]
Account [Cd, owner, balance]
Transaction [Id, account, type]
Customer Id has a feature of identity increment 1.
My goal is to create a new person for the database, so that
Customer.Id = Person.Id = Account.owner = Transaction.ID
I have tried the following below, however I get this error:
Cannot insert null value into column owner
How do I correct the mistakes to make it work?
BEGIN TRAN
BEGIN TRY
INSERT INTO bank.customer DEFAULT VALUES
INSERT INTO bank.person (id, fname, lname)
VALUES (##IDENTITY, 'Mike', 'Phelps')
INSERT INTO bank.account (cd, owner, balance)
VALUES (2, ##IDENTITY, 0)
INSERT INTO bank.transaction (id, account, type)
VALUES (##IDENTITY, (SELECT cd FROM pankki.tili,'P')
END TRY
BEGIN CATCH
ROLLBACK
SELECT
ERROR_NUMBER() AS ErrorNumber,
ERROR_STATE() AS ErrorState,
ERROR_SEVERITY() AS ErrorSeverity,
ERROR_PROCEDURE() AS ErrorProcedure,
ERROR_LINE() AS ErrorLine,
ERROR_MESSAGE() AS ErrorMessage;
END CATCH

Answer 1

I suspect what you want is this:
BEGIN TRY
BEGIN TRAN;
DECLARE #ID int; --bigint, decimal?
INSERT INTO bank.customer DEFAULT VALUES;
SET #ID = SCOPE_IDENTITY(); --Assumes customer has a column with an IDENTITY
INSERT INTO bank.person (id,fname,lname)
VALUES(#ID,'Mike','Phelps');
INSERT INTO bank.account (cd,owner,balance)
VALUES(2,#ID,0);
INSERT INTO bank.transaction(id,account,type)
SELECT #ID,
cd,
'P'
FROM pankki.tili; --I assume, therefore, that pankki.tili only ever has 1 row
COMMIT; --YOu were missing this
END TRY
BEGIN CATCH
ROLLBACK;
THROW; --Don't SELECT the error details, THROW it.
END CATCH

Answer 2

From the Microsoft document:
After an INSERT, SELECT INTO, or bulk copy statement is completed, ##IDENTITY contains the last identity value that is generated by the statement. If the statement did not affect any tables with identity columns, ##IDENTITY returns NULL.
I take it PERSON does not have an identity column, so when you insert into it, ##identity becomes NULL.
If you want to user the ##identity from the insert for the other tables, use it to set the value of a variable.
declare #PersistentID int;
INSERT INTO bank.customer DEFAULT VALUES
set #PersistentID = ##IDENTITY -- or scope_identity() is safer
INSERT INTO bank.person (id,fname,lname)
VALUES( #PersistentID ,'Mike','Phelps')

Answer 3

First, you need to understand the difference between this two commands:
##identity returns the last inserted identity value in ANY table in the current session, regardless of scope.
IDENT_CURRENT('table_or_view') returns the last inserted identity value for a GIVEN table.
So, for your case, you need to use the second one.
And your script would be something like this:
BEGIN TRAN
BEGIN TRY
INSERT INTO bank.customer DEFAULT VALUES
SET #customerID = (SELECT IDENT_CURRENT('bank.customer'))
INSERT INTO bank.person (id,fname,lname)
VALUES( #customerID,'Mike','Phelps')
INSERT INTO bank.account (cd,owner,balance)
VALUES(2,#customerID,0)
INSERT INTO bank.transaction(id,account,type)
VALUES(#customerID,(SELECT cd FROM pankki.tili,'P')
END TRY
This way you can guarantee that the same ID in inserted in the four tables.
If you are using ##identity this value is changing with every new insert.