PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Transactions
313
applications such as catalogs, MyISAM is the best choice.You should also use MyISAM if
you need full text-searching capabilities.You should use InnoDB when transactions are
important, such as for tables storing financial data or for situations in which INSERTs and
SELECTs are being interleaved, such as online message boards or forums.
You can use MEMORY tables for temporary tables or to implement views, and
MERGE tables if you need to deal with very large MyISAM tables.
You can change the type of a table after creation with an ALTER TABLE statement, as
follows:
alter table orders type=innodb;
alter table order_items type=innodb;
We used MyISAM tables through most of this part of the book.We will now spend
some time focusing on the use of transactions and ways they are implemented in
InnoDB tables.
Transactions
Transactions are mechanisms for ensuring database consistency, especially in the event of
error or server crash. In the following sections, you learn what transactions are and how
to implement them with InnoDB.
Understanding Transaction Definitions
First, let’s define the term transaction. A transaction is a query or set of queries guaranteed
either to be completely executed on the database or not executed at all.The database is
therefore left in a consistent state whether or not the transaction completed.
To see why this capability might be important, consider a banking database. Imagine
the situation in which you want to transfer money from one account to another.This
action involves removing the money from one account and placing it in another, which
would involve at least two queries. It is vitally important that either these two queries
are both executed or neither is executed. If you take the money out of one account and
the power goes out before you put it into another account, what happens? Does the
money just disappear?
You may have heard the expression ACID compliance. ACID is a way of describing
four requirements that transactions should satisfy:
n Atomicity—A transaction should be atomic; that is, it should either be completely
executed or not executed.
n Consistency—A transaction should leave the database in a consistent state.
n
n
Isolation—Uncompleted transactions should not be visible to other users of the
database; that is, until transactions are complete, they should remain isolated.
Durability—Once written to the database, a transaction should be permanent or
durable.