Revision on database
Some of DBMS engines
The SQL language
Relational algebra and translating SQL queries to relational algebra
Selection, Projection, Rename Operation
UNION Operation , Intersection, Cross product, Set Diffrence
Natural Join, ............


Transactions Processing

January 8, 2020

A Transaction is a mechanism for applying the desired modifications/operations to a database. It is evident in real life that the final database instance after a successful manipulation of the content of the database is the most up-to-date copy of the database. Action, or series of actions, carried out by a single user or application program, which accesses or changes contents of database. (i.e. Logical unit of work on the database.) A transaction could be a whole program, part/module of a program or a single command. Changes made in real time to a database are called transactions. Examples include ATM transactions, credit card approvals, flight reservations, hotel check-in, phone calls, supermarket canning, academic registration and billing.
Transaction processing system A system that manages transactions and controls their access to a DBMS is called a TP monitor. A transaction processing system (TPS) generally consists of a TP monitor, one or more DBMSs, and a set of application programs containing transaction.

State of a Transaction

Transactions can be started, attempted, then committed or aborted via data manipulation commands of SQL.
Can have one of the two outcomes for any transaction:
Success - transaction commits and database reaches a new consistent state Committed transaction cannot be aborted or rolled back.
How do you discard a committed transaction? Failure - transaction aborts, and database must be restored to consistent state before it started.
Such a transaction is rolled back or undone. Aborted transaction that is rolled back can be restarted later. In many data manipulation languages, there are keywords used to indicate different states of a transaction.

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