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You can specify a point in a transaction at which a rollback occurs by using the ROLLBACK statement. The ROLLBACK statement terminates the transaction and reverses any changes that were made up to that point. It also frees all resources held by that transaction and decrements @@TRANCOUNT to 0. If a rollback occurs in the middle of a transaction, the rest of the transaction is ignored. If the transaction encapsulates an entire stored procedure, for example, and the ROLLBACK statement occurs within the stored procedure, the stored procedure is rolled back and processing resumes at the next statement in the batch after the stored procedure call (assuming the transaction was not a nested transaction). A transaction cannot be rolled back after it commits. For an explicit rollback of a single transaction (with no nested transactions) to occur, a ROLLBACK statement must be executed before the COMMIT statement. In the case of nested transactions, once the outermost transaction has committed (which causes the inner transactions to commit also), none of the transactions can be rolled back. As mentioned previously, you cannot roll back individual inner transactions; instead, the entire outer transaction and all inner transactions are rolled back together. When the ROLLBACK statement is executed within a nested transaction, it is rolled back to the very first BEGIN TRANSATION that started the outer transaction. So, if you include a transaction name in the ROLLBACK statement, it must be the outermost transaction s name, otherwise you will receive an error from SQL Server. Below is an example of performing a ROLLBACK within a nested transaction based on the previous examples. We have added a ROLLBACK within the stored procedure, CREATE TABLE and INSERT statements in the outer transaction to use as a test for ROLLBACK and to show the effect on the outer transaction, and some print statements of @@TRANCOUNT:
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USE AdventureWorks; GO IF EXISTS (SELECT name FROM sys.objects WHERE name = N update_marital_status ) DROP PROC update_marital_status;
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GO CREATE PROCEDURE update_marital_status(@new_status char(1), @emp_id smallint, @new_title char(4)) AS DECLARE @tran_count tinyint; BEGIN TRAN update_status_tran; UPDATE HumanResources.Employee SET MaritalStatus=@new_status WHERE EmployeeID=@emp_id; UPDATE Person.Contact SET title = @new_title FROM HumanResources.Employee e JOIN Person.Contact p ON e.ContactID = p.ContactID
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WHERE e.EmployeeID = @emp_id ; SELECT @tran_count = @@TRANCOUNT; PRINT inside proc tran_count = ; PRINT @tran_count; IF (SELECT COUNT(*) from Test_Table) <> 0 ROLLBACK TRAN; ELSE COMMIT TRAN update_status_tran; GO
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DECLARE @tran_count tinyint; BEGIN TRAN outer_tran; IF EXISTS (SELECT name FROM sys.objects WHERE name = N Test_Table ) DROP TABLE Test_Table; GO CREATE TABLE Test_Table (ColA int, ColB char(1)); INSERT INTO Test_Table VALUES (1, A );
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SELECT @tran_count = @@TRANCOUNT; PRINT before proc tran_count = ; PRINT @tran_count; EXEC update_marital_status M , 8, Mrs. ; SELECT @tran_count = @@TRANCOUNT; PRINT after proc tran_count = ; PRINT @tran_count; IF @@TRANCOUNT = 1 COMMIT TRAN outer_tran; GO
Running the above code creates the stored procedure and then executes the batch (starting at the DECLARE statement) that creates the table Test_Table, inserts one row in it, executes the stored procedure that calls a rollback because the condition for the IF statement is true, and then continues with the batch. The PRINT statements show this. Because the ROLLBACK is executed, the outer transaction is rolled back as well, so at the end of the batch, the Test_Table table does not exist because the CREATE TABLE and INSERT statements are rolled back as well. To see these transactions commit, simply change the following line in the stored procedure:
IF (SELECT COUNT(*) from Test_Table) <> 0
Change to the following:
IF (SELECT COUNT(*) from Test_Table) = 0
You should then be able to view the Test_Table with one row inserted. Also note that because the ROLLBACK occurs at a different level from the BEGIN TRAN with which it corresponds (it corresponds to the outer transaction BEGIN TRAN, as we have discussed for nested transactions), SQL Server returns an error message 266. This message is expected and can be ignored, and it does not affect execution. It occurs whenever @@TRANCOUNT at the beginning of a stored procedure is different at the end of the stored procedure. If the ROLLBACK is in the outer transaction, this does not occur. More Info
For ways to avoid or work around this message, see the topic Rollbacks and Commits in Stored Procedures and Triggers in SQL Server Books Online.
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