Writing MySQL-Based Programs - MySQL - page 45

prints its own message. This is redundant, as well as confusing to the person using the
script.
If you enable RaiseError , you can call DBI methods without checking for return values
that indicate errors. If a method fails, DBI prints an error and terminates your script. If
the method returns, you can assume it succeeded. This is the easiest approach for script
writers: let DBI do all the error checking! However, if both PrintError and RaiseEr
ror are enabled, DBI may call warn() and die() in succession, resulting in error mes‐
sages being printed twice. To avoid this problem, disable PrintError whenever you
enable RaiseError :
my $conn_attrs = { PrintError => 0 , RaiseError => 1 , AutoCommit => 1 };
my $dbh = DBI -> connect ( $dsn , "baduser" , "badpass" , $conn_attrs );
This topic generally uses that approach. If you don't want the all-or-nothing behavior
of enabling RaiseError for automatic error checking versus having to do all your own
checking, adopt a mixed approach. Individual handles have PrintError and RaiseEr
ror attributes that can be enabled or disabled selectively. For example, you can enable
RaiseError globally by turning it on when you call connect() , and then disable it
selectively on a per-handle basis.
Suppose that a script reads the username and password from the command-line argu‐
ments, and then loops while the user enters statements to be executed. In this case, you'd
probably want DBI to die and print the error message automatically if the connection
fails (you cannot proceed to the statement-execution loop in that case). After connect‐
ing, however, you wouldn't want the script to exit just because the user enters a syntac‐
tically invalid statement. Instead, print an error message and loop to get the next state‐
ment. The following code shows how to do this. The do() method used in the example
executes a statement and returns undef to indicate an error:
my $user_name = shift ( @ARGV );
my $password = shift ( @ARGV );
my $conn_attrs = { PrintError => 0 , RaiseError => 1 , AutoCommit => 1 };
my $dbh = DBI -> connect ( $dsn , $user_name , $password , $conn_attrs );
$dbh -> { RaiseError } = 0 ; # disable automatic termination on error
print "Enter statements to execute, one per line; terminate with Control-D\n" ;
while ( <> ) # read and execute queries
{
$dbh -> do ( $_ ) or warn "Statement failed: $DBI::errstr ($DBI::err)\n" ;
}
If RaiseError is enabled, you can execute code within an eval block to trap errors
without terminating your program. If an error occurs, eval returns a message in the $@
variable:
eval
{
# statements that might fail go here...
};