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A transaction is a single unit of work. If a transaction is successful, all of the data modifications made during the transaction are committed and become a permanent part of the database. If a transaction encounters errors and must be canceled or rolled back, then all of the data modifications are erased.
The following are some frequent scenarios where use of transactions is recommended: In batch processing, where multiple rows must be inserted, updated, or deleted as a single unit. Whenever a change to one table requires that other tables be kept consistent. When modifying data in two or more databases concurrently.
Transactions that span multiple statements leave locks that hurt concurrency. So "always" creating a transactions is not a good idea. You should balance the cost against the benefit. "A SQL statement always runs in a transaction".
Add a try/catch block, if the transaction succeeds it will commit the changes, if the transaction fails the transaction is rolled back:
BEGIN TRANSACTION [Tran1]
BEGIN TRY
INSERT INTO [Test].[dbo].[T1] ([Title], [AVG])
VALUES ('Tidd130', 130), ('Tidd230', 230)
UPDATE [Test].[dbo].[T1]
SET [Title] = N'az2' ,[AVG] = 1
WHERE [dbo].[T1].[Title] = N'az'
COMMIT TRANSACTION [Tran1]
END TRY
BEGIN CATCH
ROLLBACK TRANSACTION [Tran1]
END CATCH
At the beginning of stored procedure one should put SET XACT_ABORT ON to instruct Sql Server to automatically rollback transaction in case of error. If ommited or set to OFF one needs to test @@ERROR after each statement or use TRY ... CATCH rollback block.
Easy approach:
CREATE TABLE T
(
C [nvarchar](100) NOT NULL UNIQUE,
);
SET XACT_ABORT ON -- Turns on rollback if T-SQL statement raises a run-time error.
SELECT * FROM T; -- Check before.
BEGIN TRAN
INSERT INTO T VALUES ('A');
INSERT INTO T VALUES ('B');
INSERT INTO T VALUES ('B');
INSERT INTO T VALUES ('C');
COMMIT TRAN
SELECT * FROM T; -- Check after.
DELETE T;
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