Learning MySQL

Handling Errors When Interacting with the Database

If the connection to the database fails, the dbh variable will contain an undefined (effectively false) value. We should test for this and stop the program if the connection failed; otherwise, we’ll run into difficulties once the program tries to use the database, generating unhelpful error messages such as:

Can't call method "prepare" on an undefined value at ./select.pl line 9.

One way to test for connection failure is simply to check the value of the database handler variable:

my $dbh=DBI->connect("DBI:mysql:host=localhost;database=$DB_Database",
 "$DB_Username", "$DB_Password");
if(!$dbh)
{
 die("Failed connecting to the database.");
}

If the database handler is not valid, we use the die() function to print an error message and stop the program.

A more compact way is to use the or keyword to execute the die() function if the connection failed:

my $dbh=DBI->connect("DBI:mysql:host=localhost;database=$DB_Database",
 "$DB_Username", "$DB_Password")
or
 die("Failed connecting to the database.");

Finally, we can modify problem-handling behavior by setting the attributes PrintError and RaiseError in the call to the connect() function. Setting the PrintError attribute to 1 displays error messages; setting it to 0 disables this. Similarly, setting the RaiseError attribute to 1 displays an error message and stops processing if an error occurs; setting it to 0 disables this. If both are set to 0, no error messages are displayed, and the program ...