dbi: the neophyte's guide1 what is dbi? dbi = database interface dbi is database-independent...
DESCRIPTION
DBI: The Neophyte's Guide3 Why do we need the DBI? Abstracted interface hides database-specific functionality from the programmer DBI scripts are generally completely portable between databases with little modification Faster application development and debugging cycleTRANSCRIPT
DBI: The Neophyte's Guide 1
What is DBI?
• DBI = DataBase Interface• DBI is database-independent• DBI allows you to write code that interacts with
databases independent of the underlying database• A DBI program will work with little, or no,
modification on Oracle, Informix, MySQL and so on...
DBI: The Neophyte's Guide 2
What are DBDs?
• DBD = DataBase Driver• Drivers are used by DBI to perform the actual
database work• Drivers are database-dependent• Drivers exist for most popular databases
including Oracle, Informix, Ingres, mSQL, MySQL, Solid and many more
DBI: The Neophyte's Guide 3
Why do we need the DBI?
• Abstracted interface hides database-specific functionality from the programmer
• DBI scripts are generally completely portable between databases with little modification
• Faster application development and debugging cycle
DBI: The Neophyte's Guide 4
What about ODBC?• ODBC and DBI share similar goals and heritage:
platform- and database-independence• DBI is far more compact and less complex than
ODBC. ODBC has a richer feature set• Although ODBC drivers exist for many operating
systems, they can be expensive and difficult to use. DBI is free and easy to configure and use
• A driver called DBD::ODBC exists and can be used to connect to ODBC-based databases such as Microsoft Access
DBI: The Neophyte's Guide 5
DBI Resources
• The DBI home page– http://www.symbolstone.org/technology/perl/DBI
• ``Programming the Perl DBI’’ -- Alligator Descartes and Tim Bunce. Can be ordered via amazon.com from the DBI home page
• This presentation and demonstration source code can be downloaded from the DBI home page
DBI: The Neophyte's Guide 6
DBI Architecture• Architectural Overview• A Handy Reminder• Handles
– Driver Handles– Database Handles– Statement Handles
• Basic Database Operations– Connecting to the Database
• Data Sources– Disconnecting from the Database
DBI: The Neophyte's Guide 7
Architectural Overview
DBI: The Neophyte's Guide 8
A Handy Reminder
• A tip to help you remember which modules do what in DBI
– DBI is DataBase Independent– DBD is DataBase Dependent
DBI: The Neophyte's Guide 9
Handles
• All operations in DBI are performed via– DBI module as a static method– Driver Handles– Database Handle instance method– Statement Handle instance method
• Driver Handles are not used explicitly by the programmer in scripts
DBI: The Neophyte's Guide 10
Driver Handles
• Driver Handles encapsulate a DBD• There exists exactly one driver handle for each
loaded DBD• For example, a script using both MySQL and
Oracle will have two instantiated driver handles• Driver handles are never used directly in
programs and are for internal use only by DBI
DBI: The Neophyte's Guide 11
Database Handles
• Database Handles encapsulate a single connection to a database
• Database handles are created via a driver handle for the desired database type. This is done via the DBI->connect() method $dbh = DBI->connect( … );
DBI: The Neophyte's Guide 12
Statement Handles
• Statement Handles encapsulate a statement to be issued to the database
• Statement handles are created via a database handle. That is, a statement is issued to the database represented by the database handle$sth = $dbh->prepare( “SELECT yadda FROM blah” );
• Statement handles also allow you to fetch data from the issued statement from the database
DBI: The Neophyte's Guide 13
Handle Naming Conventions
• Throughout the example code in this presentation and other DBI texts, you might see the following naming convention of variables– $drh = Driver Handle– $dbh = Database Handle– $sth = Statement Handle
DBI: The Neophyte's Guide 14
Starting Out with DBI
• Initializing the DBI • Data Sources• Checking Available Drivers• Connecting to the Database• Disconnecting from the Database• Example
DBI: The Neophyte's Guide 15
Initializing the DBI
• DBI can be initialized very simply by adding
use DBI;
to your scripts• DBI internally handles all driver loading
automatically and DBDs should not normally be explicitly use’d or require’d in your scripts
DBI: The Neophyte's Guide 16
Data Sources
• In summary, Data Source specification is about the most non-portable aspect of DBI
• In reality, a well-written DBI script that uses standard SQL can be ported by simply changing the DSN in the DBI->connect() call
DBI: The Neophyte's Guide 17
Connecting to the Database• The very first database operation you must do!• Use the DBI->connect()method• For example $dbh = DBI->connect( ‘dbi:Oracle:DEV’, ‘user’, ‘pass’ );
• That is, we minimally specify the Data Source Name of the database, a username and password
• If successful, this will return a valid database handle
DBI: The Neophyte's Guide 18
Disconnecting from the Database
• Once all work has been completed, you must disconnect from the database
• Frees any used database and system resources• Achieved by calling disconnect() against a valid
database handle. For example: $dbh->disconnect();
DBI: The Neophyte's Guide 19
Example • This example connects to a single Oracle database called
DEV
#!/usr/bin/perl -w
### Load the DBI moduleuse DBI; ### Perform the connection using the Oracle driver$dbh = DBI->connect( "dbi:Oracle:DEV", "username", "password" ) or die "Can't connect to Oracle database: $DBI::errstr\n"; ### Disconnect from the database$dbh->disconnect();
exit;
DBI: The Neophyte's Guide 20
Example • This example demonstrates connecting to two
different databases of different types#!/usr/bin/perl -w
### Load the DBI module use DBI; ### Perform the connection using the Oracle driver$dbh1 = DBI->connect( "dbi:Oracle:DEV", "username", "password" ) or die "Can't connect to Oracle database: $DBI::errstr\n"; $dbh2 = DBI->connect( "dbi:mSQL:host:dbname:1114", "username", "password" ) or die "Can't connect to mSQL database: $DBI::errstr\n"; $dbh1->disconnect();$dbh2->disconnect();
exit;
DBI: The Neophyte's Guide 21
Interacting with the Database• Simple Queries
– Preparing Statements– Executing Statements– Fetching Data
• Non-SELECT Statements• Bind Values• Optimizing $dbh->do()
DBI: The Neophyte's Guide 22
Simple Queries
• The process of retrieving data from the database is a 4-stage cycle– Preparing the statement– Executing the statement– Fetching the data– Finishing the statement
DBI: The Neophyte's Guide 23
Preparing the Statement• The $dbh->prepare( $statement ) method is
used• This can mean different things depending on the
underlying DBD used. Oracle will use this stage to send the statement to the database for parsing. However, mSQL does nothing here at all
• The SQL statement is simply a Perl string and therefore can be supplied as a literal string or as a variable holding a built on-the-fly statement
DBI: The Neophyte's Guide 24
Preparing the Statement ( cont. )#!/usr/bin/perl -w
### Load the DBI moduleuse DBI; ### The database handle$dbh = DBI->connect( "dbi:Oracle:DEV", "username", "password" );
### Prepare the statement handle$sth = $dbh->prepare( "SELECT id, name FROM megaliths" );
...
exit;
DBI: The Neophyte's Guide 25
Executing the Statement
• Once the statement has been successfully prepared, it must be executed
• Depending on the database, this stage typically executes the prepared statement within the database and generates a result set ready for fetching
• Simply a case of calling $sth->execute() against your prepared statement handle
DBI: The Neophyte's Guide 26
Fetching the Data
• The fetch stage retrieves the data from the database row-by-row by use of a cursor
• The most commonly used fetch style is to retrieve the data as a Perl list via @row = $sth->fetchrow_array()
• You should continuously loop calling fetchrow_array() until all rows are returned
DBI: The Neophyte's Guide 27
Fetching Data - Example #/usr/bin/perl -w
### Load the DBI moduleuse DBI;
### Connect to the database$dbh = DBI->connect( ‘dbi:Oracle:DEV’, ‘username’, ‘password’ );
### Prepare and execute the statement$sth = $dbh->prepare( “SELECT name, type FROM megaliths” );$sth->execute();
### Fetch the datawhile ( @row = $sth->fetchrow_array() ) { print "Megalith site $row[0] is a $row[1]\n"; }
### Disconnect from the database$dbh->disconnect();
exit;
DBI: The Neophyte's Guide 28
Fetching the Data
• There are additional methods for fetching data from the statement handle:
fetchrow_arrayref()fetchrow_hashref()
DBI: The Neophyte's Guide 29
Finishing the Statement
• DBI automatically marks the statement handles as inactive once all the data has been fetched from it
• You can manually mark statement handles as being inactive via the $sth->finish() method, but you should usually not need to invoke this method in your scripts
DBI: The Neophyte's Guide 30
Non-SELECT Statements
• Statements other than queries can be issued to the database via DBI
• For example, UPDATE, DELETE and INSERT statements
• These statements are issued by calling $dbh->do()
which combines preparation and execution in a single step
DBI: The Neophyte's Guide 31
Non-SELECT Statements ( cont. )
• For example:
$dbh->do( “DELETE FROM megaliths” );$dbh->do( “DROP TABLE megaliths” );$dbh->do( “INSERT INTO megaliths VALUES ( 1, ‘Stonehenge’, ‘Wiltshire’ )” );
DBI: The Neophyte's Guide 32
Bind Values
• A bind value is a value that can be bound to a placeholder declared within a SQL statement
• This is similar to creating an on-the-fly SQL statement such as:
$sth = $dbh->prepare( " SELECT name, location FROM megaliths WHERE name = " . $dbh->quote( $siteName ) . " " );
DBI: The Neophyte's Guide 33
Bind Values ( cont. )• However, instead of interpolating the generated
value into the SQL statement, you specify a placeholder and then bind the generated value to that
• For example: $sth = $dbh->prepare( " SELECT name, location FROM megaliths WHERE name = ? " ); $sth->bind_param( 1, $dbh->quote( ‘Avebury’ ) );
DBI: The Neophyte's Guide 34
Optimizing $dbh->do()
• The default implementation of $dbh->do() calls prepare(), execute() and finish()for each invocation of $dbh->do()
• This can be optimized for repeated iterations by hand-preparing the statement but repeatedly executing the same statement handle
• This is very efficient when used with bind values
DBI: The Neophyte's Guide 35
Optimizing $dbh->do() ### Prepare the statement handle $sth = $dbh->prepare( “DELETE FROM megaliths WHERE id = ?” );
### Remove the first 100 rows one-by-one... $loopCounter = 0; while ( $loopCounter < 100 ) { $sth->execute( $loopCounter ); $loopCounter++; }
$sth->finish();
...
• This is far faster than repeatedly preparing the same statement over and over again