Introduction
The examples used in this article uses the Pubs database that comes as a
sample database when you install SQL Server. If you need to rebuild the Pubs
database, follow the steps to install a fresh copy :
- Run the osql command prompt utility and detach the Pubs database from SQL
Server by using the sp_detach_db system stored procedure.
osql -U sa -P "" -Q "exec sp_detach_db 'Pubs'"
- Delete the database files for pubs database (pubs.mdf, pubs_log.ldf). These
files are located in the \Data directory.
- Re-creating the Pubs database requires the Instpubs.sql script to be
executed. Run the script from the command line (if the .sql files are in a
different directory, adjust the path accordingly). You can also run this script
file from the Query Analyzer.
osql -U sa -P "" -i
"C:\Program Files\Microsoft SQL Server\MSSQL\Install\InstPubs.sql"
(The osql utility uses case-sensitive options. If neither the -U or -P
options are used, SQL Server 2000 attempts to connect using Windows
Authentication Mode. More information about the osql Utility can be found in the
Sql Server Books Online)
Transactions
Transactions group a set of tasks into a single execution unit. Each
transaction begins with a specific task and ends when all the tasks in the group
successfully complete. If any of the tasks fails, the transaction fails.
Therefore, a transaction has only two results: success or failure. Incomplete
steps result in the failure of the transaction.
Users can group two or more Transact-SQL statements into a single transaction
using the following statements:
- Begin Transaction
- Rollback Transaction
- Commit Transaction
If anything goes wrong with any of the grouped statements, all changes need
to be aborted. The process of reversing changes is called rollback in SQL
Server terminology. If everything is in order with all statements within a
single transaction, all changes are recorded together in the database. In SQL
Server terminology, we say that these changes are committed to the
database.
Here is an example of a transaction :
USE pubs
DECLARE @intErrorCode INT
BEGIN TRAN
UPDATE Authors
SET Phone = '415 354-9866'
WHERE au_id = '724-80-9391'
SELECT @intErrorCode = @@ERROR
IF (@intErrorCode <> 0) GOTO PROBLEM
UPDATE Publishers
SET city = 'Calcutta', country = 'India'
WHERE pub_id = '9999'
SELECT @intErrorCode = @@ERROR
IF (@intErrorCode <> 0) GOTO PROBLEM
COMMIT TRAN
PROBLEM:
IF (@intErrorCode <> 0) BEGIN
PRINT 'Unexpected error occurred!'
ROLLBACK TRAN
END
Before the real processing starts, the BEGIN TRAN statement
notifies SQL Server to treat all of the following actions as a single
transaction. It is followed by two UPDATE statements. If no errors
occur during the updates, all changes are committed to the database when SQL
Server processes the COMMIT TRAN statement, and finally the stored
procedure finishes. If an error occurs during the updates, it is detected by if
statements and execution is continued from the PROBLEM label. After
displaying a message to the user, SQL Server rolls back any changes that
occurred during processing. Note: Be sure to match BEGIN TRAN with
either COMMIT or ROLLBACK.
Nested Transactions
SQL Server allows you to nest transactions. Basically, this feature means
that a new transaction can start even though the previous one is not complete.
Transact-SQL allows you to nest transaction operations by issuing nested
BEGIN TRAN commands. The @@TRANCOUNT automatic
variable can be queried to determine the level of nesting - 0 indicates no
nesting , 1 indicates nesting one level deep, and so fourth.
A COMMIT issued against any transaction except the outermost one
doesn't commit any changes to disk - it merely decrements the@@TRANCOUNT
automatic variable. A ROLLBACK, on the other hand, works
regardless of the level at which it is issued, but rolls back all transactions,
regardless of the nesting level. Though this is counterintuitive, there's a very
good reason for it. If a nested COMMIT actually wrote changes
permanently to disk, an outer ROLLBACK wouldn't be able to reverse
those changes since they would already be recorded permanently.
When you explicitly begin a transaction, the @@TRANCOUNT
automatic variable count increases from 0 to 1; when you
COMMIT, the count decreases by one; when you ROLLBACK,
the count is reduced to 0. As you see, the behavior of COMMIT and
ROLLBACK is not symmetric. If you nest transactions, COMMIT
always decreases the nesting level by 1, as you can see illustrated in
Figure 1. The ROLLBACK command, on the other hand, rolls back the
entire transaction, illustrated in Figure 2. This asymmetry between COMMIT
and ROLLBACK is the key to handling errors in nested
transactions.
 |
| Figure 1: A COMMIT always balances a BEGIN TRANSACTION
by reducing the transaction count by one. |
| |
 |
| Figure 2: A single ROLLBACK always rolls back the entire
transaction. |
As you can see from Figure 1 and Figure 2, you can nest transactions and use
the @@TRANCOUNT automatic variable to detect the level. You also
learned that COMMIT and ROLLBACK do not behave
symmetrically; COMMIT just decreases the value of
@@TRANCOUNT, while ROLLBACK resets it to 0. The
implication is that a transaction is never fully committed until the last
COMMIT is issued. No matter how deeply you nest a set of
transactions, only the last COMMIT has any effect.
Here is an example of a nested transaction :
USE pubs
SELECT 'Before BEGIN TRAN', @@TRANCOUNT
BEGIN TRAN
SELECT 'After BEGIN TRAN', @@TRANCOUNT
DELETE sales
BEGIN TRAN nested
SELECT 'After BEGIN TRAN nested', @@TRANCOUNT
DELETE titleauthor
COMMIT TRAN nested
SELECT 'After COMMIT TRAN nested', @@TRANCOUNT
ROLLBACK TRAN
SELECT 'After ROLLBACK TRAN', @@TRANCOUNT
SELECT TOP 5 au_id FROM titleauthor
In this example we see that despite the nested COMMIT TRAN, the
outer ROLLBACK still reverses the effects of the DELETE
titleauthor command.
Here is another similar example of nested transaction :
USE pubs
SELECT 'Before BEGIN TRAN', @@TRANCOUNT
BEGIN TRAN
SELECT 'After BEGIN TRAN', @@TRANCOUNT
DELETE sales
BEGIN TRAN nested
SELECT 'After BEGIN TRAN nested', @@TRANCOUNT
DELETE titleauthor
ROLLBACK TRAN
SELECT 'After COMMIT TRAN nested', @@TRANCOUNT
IF (@@TRANCOUNT > 0) BEGIN
COMMIT TRAN
SELECT 'After COMMIT TRAN', @@TRANCOUNT
END
SELECT TOP 5 au_id FROM titleauthor
In this example, execution never reaches the out COMMIT TRAN
because the ROLLBACK TRAN reverses all
transactions currently in progress and sets @@TRANCOUNT to 0.
Unless ROLLBACK TRAN is called with a save point, ROLLBACK
TRAN always rolls back all transactions and sets @@TRANCOUNT
to 0, regardless of the context in which it's called.
SAVE TRAN and Save Points
Savepoints offer a mechanism to roll back portions of transactions. A user
can set a savepoint, or marker, within a transaction. The savepoint defines a
location to which a transaction can return if part of the transaction is
conditionally canceled. SQL Server allows you to use savepoints via the
SAVE TRAN statement, which doesn't affect the @@TRANCOUNT
value. A rollback to a savepoint (not a transaction) doesn't affect the
value returned by @@TRANCOUNT, either. However, the rollback must
explicitly name the savepoint: using ROLLBACK TRAN without a
specific name will always roll back the entire transaction.
The following script demonstrates how savepoints can be used :
USE pubs
SELECT 'Before BEGIN TRAN main', @@TRANCOUNT
BEGIN TRAN main
SELECT 'After BEGIN TRAN main', @@TRANCOUNT
DELETE sales
SAVE TRAN sales
SELECT 'After SAVE TRAN sales', @@TRANCOUNT
BEGIN TRAN nested
SELECT 'After BEGIN TRAN nested', @@TRANCOUNT
DELETE titleauthor
SAVE TRAN titleauthor
SELECT 'After SAVE TRAN titleauthor', @@TRANCOUNT
ROLLBACK TRAN sales
SELECT 'After ROLLBACK TRAN sales', @@TRANCOUNT
SELECT TOP 5 au_id FROM titleauthor
IF (@@TRANCOUNT > 0) BEGIN
ROLLBACK TRAN
SELECT 'AFTER ROLLBACK TRAN', @@TRANCOUNT
END
SELECT TOP 5 au_id FROM titleauthor
Error Handling
The examples presented here are specific to stored procedures as they are the
desired method of interacting with a database. When an error is encountered
within a stored procedure, the best you can do is halt the sequential processing
of the code and either branch to another code segment in the procedure or return
processing to the calling application. The @@ERROR automatic
variable is used to implement error handling code. It contains the error ID
produced by the last SQL statement executed during a client�s connection. When a
statement executes successfully, @@ERROR contains 0. To determine
if a statement executes successfully, an IF statement is used to check the value
of @@ERROR immediately after the target statement executes. It is
imperative that @@ERROR be checked immediately after the target
statement, because its value is reset to 0 when the next statement executes
successfully. If a trappable error occurs, @@ERROR will have a
value greater than 0. SQL Server resets the @@ERROR value after
every successful command, so you must immediately capture the @@ERROR
value. Most of the time, you'll want to test for changes in @@ERROR
right after any INSERT, UPDATE, or DELETE
statement.
CREATE PROCEDURE addTitle(@title_id VARCHAR(6), @au_id VARCHAR(11),
@title VARCHAR(20), @title_type CHAR(12))
AS
BEGIN TRAN
INSERT titles(title_id, title, type)
VALUES (@title_id, @title, @title_type)
IF (@@ERROR <> 0) BEGIN
PRINT 'Unexpected error occurred!'
ROLLBACK TRAN
RETURN 1
END
INSERT titleauthor(au_id, title_id)
VALUES (@au_id, @title_id)
IF (@@ERROR <> 0) BEGIN
PRINT 'Unexpected error occurred!'
ROLLBACK TRAN
RETURN 1
END
COMMIT TRAN
RETURN 0
This kind of solution contains substantial repetition especially if your
business logic requires more than two Transact-SQL statements to be implemented.
A more elegant solution is to group codes into a generic error handling
procedure:
CREATE PROCEDURE addTitle(@title_id VARCHAR(6), @au_id VARCHAR(11),
@title VARCHAR(20), @title_type CHAR(12))
AS
BEGIN TRAN
INSERT titles(title_id, title, type)
VALUES (@title_id, @title, @title_type)
IF (@@ERROR <> 0) GOTO ERR_HANDLER
INSERT titleauthor(au_id, title_id)
VALUES (@au_id, @title_id)
IF (@@ERROR <> 0) GOTO ERR_HANDLER
COMMIT TRAN
RETURN 0
ERR_HANDLER:
PRINT 'Unexpected error occurred!'
ROLLBACK TRAN
RETURN 1
