Why the following t-sql transaction does not work as expected? - sql-server

Question

To test how a transaction work, I wrote the following t-sql. As the first attribute is the primary key, none of the insertions should be committed.
But, the first insertion is committed? Why?
begin transaction
insert into instructor
values ('99999', 'Yellow', 'Biology', 700000)
insert into instructor
values ('99999', 'Blue', 'Statistics', 85000)
commit;
select * from instructor where ID = '99999'
--delete from instructor where ID = '99999'
(1 row affected) Msg 2627, Level 14, State 1, Line 100 Violation of
PRIMARY KEY constraint 'PK__instruct__3214EC278C8DA99F'. Cannot insert
duplicate key in object 'dbo.instructor'. The duplicate key value is
(99999). The statement has been terminated.
Completion time: 2019-11-06T14:02:27.3436411+02:00

Answer 1

Because, when the XACT_ABORT IS OFF (which is the default):
When SET XACT_ABORT is OFF, in some cases only the Transact-SQL
statement that raised the error is rolled back and the transaction
continues processing.
and when it is ON:
.. if a Transact-SQL statement raises a run-time error, the entire
transaction is terminated and rolled back.
That's what we need to turn it ON and if you try the code below, you can check this:
DROP TABLE IF EXISTS [dbo].[StackOverflow];
CREATE TABLE [dbo].[StackOverflow]
(
[StackID] TINYINT PRIMARY KEY
);
SET XACT_ABORT ON;
BEGIN TRANSACTION;
INSERT INTO [dbo].[StackOverflow] ([StackID])
VALUES (105);
INSERT INTO [dbo].[StackOverflow] ([StackID])
VALUES (105);
COMMIT TRANSACTION;
SET XACT_ABORT OFF;
SELECT [StackID]
FROM [dbo].[StackOverflow];
Also, note that:
Compile errors, such as syntax errors, are not affected by SET XACT_ABORT.
The above means, that if you want to really have automatic transaction you need to use the following code block:
SET NOCOUNT, XACT_ABORT ON;
BEGIN TRY
BEGIN TRANSACTION;
-- CODE BLOCK GOES HERE
COMMIT TRANSACTION;
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0
BEGIN
ROLLBACK TRANSACTION
END;
-- GET ERRORS DETAILS OR THROW ERROR
END CATCH;
SET NOCOUNT, XACT_ABORT OFF;
If you do not use TRY-CATCH block in a specific situations, like the next one, the first statement will be again committed:
DROP TABLE IF EXISTS [dbo].[StackOverflow];
CREATE TABLE [dbo].[StackOverflow]
(
[StackID] TINYINT PRIMARY KEY
);
SET XACT_ABORT ON;
BEGIN TRANSACTION;
INSERT INTO [dbo].[StackOverflow] ([StackID])
VALUES (105);
EXEC
(
'INSERrrrrrT INTO [dbo].[StackOverflow] ([StackID]) VALUES (106);'
)
COMMIT TRANSACTION;
SET XACT_ABORT OFF;
SELECT [StackID]
FROM [dbo].[StackOverflow];

Answer 2

You need to add SET XACT_ABORT ON; and/or use TRY/CATCH so that the script doesn't continue to the COMMIT after an error.
SET XACT_ABORT ON example:
SET XACT_ABORT ON;
begin transaction
insert into instructor
values ('99999', 'Yellow', 'Biology', 700000)
insert into instructor
values ('99999', 'Blue', 'Statistics', 85000)
commit;
TRY/CATCH example:
BEGIN TRY
begin transaction
insert into instructor
values ('99999', 'Yellow', 'Biology', 700000)
insert into instructor
values ('99999', 'Blue', 'Statistics', 85000)
commit;
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0 ROLLBACK;
THROW;
END CATCH;
Although SET XACT_ABORT ON is optional with this TRY/CATCH pattern, I still recommend it in all T-SQL code with explict transactions. This will ensure the transaction is rolled back immediately even when batch is cancelled manually or due to a query timeout where no CATCH block code executes. Otherwise, the uncommitted transaction will remain open until the connection is closed or reused with connection pooling.