This document discusses database transactions in MySQL. It defines a transaction as a logical unit of work that maintains data integrity. Transactions have ACID properties - atomicity, consistency, isolation, and durability. MySQL runs with implicit transactions by default, but explicit transactions can be defined using START TRANSACTION, COMMIT, and ROLLBACK commands. Transactions allow multiple statements to be executed atomically and ensure data integrity even if errors occur.

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4. TRANSACTIONS IN MYSQL
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5. WHAT IS TRANSACTION?
 Many applications require a lot of users to access the data
simultaneously (e.g. airline booking systems)
 Uncontrolled simultaneous access can result in inconsistency, so
some controlling mechanism is required
 A transaction is a logical unit of work which takes the DB from one
consistent state to another, i.e. obeying constraints
 It will probably be made up of smaller operations which temporarily
cause inconsistency

6. DATABASE TRANSACTION
Database transactions are logical units of work
which must ALL be performed to maintain data
integrity
E.g. Move money from one account to another
UPDATE Account SET balance = balance – 100
WHERE accountNo = 123;
UPDATE Account SET balance = balance + 100
WHERE accountNo = 124;

7. DATABASE TRANSACTION
 A transaction is the execution of a program
that accesses the DB and
starts with a BEGIN operation,
followed by a sequence of READ and WRITE operations,
ending with a COMMIT operation.
 An update, for example adding 10 to a value,
will actually
begin
first read the value,
calculate the new value,
and then write the new value
commit

8. DATABASE TRANSACTION
 Transactions are used for three purposes in DBMS:
 To determine when integrity constraint checks
should occur (only at the end of transactions)
 To control concurrent access. Gives a single user
the illusion of being the sole user of the database
 To manage recovery from system crashes

9. ACID PROPERTIES OF TRANSACTIONS
Atomicity
 ALL operations in a transaction must be completed. If not, the
transaction is aborted. The entire transaction is treated as a
single, indivisible unit of work which must be performed
completely or not at all.
 Consistency
 A successful transaction takes the database from one state that is
consistent with the rules to another state that is also consistent
with the rules.
 If an operation is executed that violates the database’s integrity
constraints, the entire transaction will be rolled back.

10. ACID PROPERTIES OF TRANSACTIONS
Isolation
 Data used within a transaction cannot be used by another
transaction until the first transaction is completed. (or it must
appear that this happened!). The partial effects of incomplete
transactions should not be visible to other transactions.
Durability
 Once the transaction changes have been made, they will survive
failure. The recovery system must ensure this.

11. TRANSACTIONS IN MYSQL
To successfully transfer money, use the START TRANSACTION and
COMMIT commands:
START TRANSACTION
UPDATE Account SET balance = balance – 100
WHERE accountNo = 123;
UPDATE Account SET balance = balance + 100
WHERE accountNo = 124;
COMMIT;
The database is not updated until the COMMIT command is executed

12. TRANSACTIONS IN MYSQL
 MySQL runs by default with auto-commit enabled.
Each MySQL statement is treated as a single transaction, with an
implicit COMMIT at the end.
In this case,
UPDATE Account SET balance = balance + 100;
is the same as
START TRANSACTION
UPDATE Account SET balance = balance + 100;
COMMIT;

13. ROLL BACK
The DBMS maintains a transaction log.
If the computer crashes in the middle of a transaction,
the DBMS will rollback the database to the last
completed transaction

14. ROLL BACK
Also, you can use the MySQL ROLLBACK command
This is most useful for testing updates – your database is
restored to the state immediately before the last
transaction. Check it worked, then rollback
START TRANSACTION
UPDATE Account SET bal = bal – 100 WHERE
accountNo = 123;
UPDATE Account SET bal = bal + 100 WHERE
accountNo = 124;
SELECT balance FROM Account WHERE
accountNo = 123;
SELECT balance FROM Account WHERE
accountNo = 124;
ROLLBACK;

15. Pk_student_id Fk_teacher_id Student_course
Pk_teacher_id Teacher_name
ANOTHER EXAMPLE

16. ANOTHER EXAMPLE
Create procedure sample()
Begin
Set a int defaul 0;
Set b int default 0;
START TRANSACTION;
Insert into tbl_teachers (teacher_name) values(’john’);
Set a = last_insert_id();
Insert into Tbl_student (Fk_teacher_id,student_course) values(1,’php’);
Set b = last_insert_id();
If a>0 && b>0 THEN
COMMIT;
ELSE
ROLLBACK;
End

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