Transaction is very interesting topic, i have came across ever.It is very
necessary for maintaining integrity of database.
So let’s
see some interesting facts about transactions.
A transaction is a unit of work that is performed
against a database. Transactions are units or sequences of work accomplished in
a logical order, whether in a manual fashion by a user or automatically by some
sort of a database program.
For example, if you are creating a record or updating a
record or deleting a record from the table, then you are performing transaction
on the table. It is important to control transactions to ensure data integrity
and to handle database errors.
Practically, you will club many SQL queries into a
group and you will execute all of them together as a part of a transaction.
Properties
of Transactions:
Transactions have the following four standard
properties, usually referred to by the acronym ACID:
Atomicity:
ensures that all operations within the work unit are completed successfully;
otherwise, the transaction is aborted at the point of failure, and previous operations
are rolled back to their former state.
Consistency:
ensures that the database properly changes states upon a successfully committed
transaction.
Isolation:
enables transactions to operate independently of and transparent to each other.
Durability:
ensures that the result or effect of a committed transaction persists in case
of a system failure.
Transaction
Control:
There are following commands used to control
transactions:
COMMIT: to save the changes.
ROLLBACK: to rollback the changes.
SAVEPOINT: creates points within groups of
transactions in which to ROLLBACK
SET TRANSACTION: Places a name on a transaction.
Transactional control commands are only used with the
DML commands INSERT, UPDATE and DELETE only. They can not be used while creating
tables or dropping them because these operations are automatically committed in
the database.
The
COMMIT Command:
The COMMIT command is the transactional command used to
save changes invoked by a transaction to the database.
The COMMIT command saves all transactions to the
database since the last COMMIT or ROLLBACK command.
The syntax for COMMIT command is as follows:
COMMIT;
The
ROLLBACK Command:
The ROLLBACK command is the transactional command used
to undo transactions that have not already been saved to the database.
The ROLLBACK command can only be used to undo
transactions since the last COMMIT or ROLLBACK command was issued.
The syntax for ROLLBACK command is as follows:
ROLLBACK;
The
SAVEPOINT Command:
A SAVEPOINT is a point in a transaction when you can
roll the transaction back to a certain point without rolling back the entire
transaction.
The syntax for SAVEPOINT command is as follows:
SAVEPOINT
SAVEPOINT_NAME;
This command serves only in the creation of a SAVEPOINT
among transactional statements. The ROLLBACK command is used to undo a group of
transactions.
The syntax for rolling back to a SAVEPOINT is as
follows:
ROLLBACK
TO SAVEPOINT_NAME;
The
RELEASE SAVEPOINT Command:
The RELEASE SAVEPOINT command is used to remove a
SAVEPOINT that you have created.
The syntax for RELEASE SAVEPOINT is as follows:
RELEASE
SAVEPOINT SAVEPOINT_NAME;
Once a SAVEPOINT has been released, you can no longer
use the ROLLBACK command to undo transactions performed since the SAVEPOINT.
The
SET TRANSACTION Command:
The SET TRANSACTION command can be used to initiate a
database transaction. This command is used to specify characteristics for the
transaction that follows.
For example, you can specify a transaction to be read
only, or read write.
The syntax for SET TRANSACTION is as follows:
SET
TRANSACTION [ READ WRITE | READ ONLY ];
Your comments and suggestions are appreciated....
Thanks..!!! :)
