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14.11. Dealing with Database Errors
14.11.1. Problem
You want your program to catch and
handle database errors, possibly displaying informative error
messages.
14.11.2. Solution
The best solution is to enable RaiseError when you connect to the
database, then wrap database calls in eval:
$dbh = DBI->connect($DSN, $user, $password,
{ RaiseError > 1 });
eval {
$dbh->do($SQL);
$sth = $dbh->prepare($SQL2);
$sth->execute( );
while (@row = $sth->fetchrow_array) {
# ...
}
};
if ($@) {
# recover here using $DBI::lasth->errstr to get
# the error message
}
14.11.3. Discussion
The logic here is simple: first tell DBI to die if
there's a problem with your SQL (otherwise, the database can't do
what you wanted it to). Then, wrap the code that might
die in eval to catch fatal
errors. Next, check $@ (either the error message
you would have die d with or empty if there was no
error) to see whether something went wrong. If it did, somehow deal
with the error.DBI supplies the $DBI::lasth variable, containing
the last handle acted on. If something went wrong, that's the handle
that caused it. You could use the $@ error
message, but that also has the "died at file ... line ..." text from
die that you might not want. To discover the SQL
statement that died, use
$DBI::lasth->{Statement}. If you are using only
the one handle, you can call the methods directly on your handle
instead of on $DBI::lasth:
$msg = $dbh->errstr;
$sql = $dbh->{Statement};
An alternative approach is to disable RaiseError and check the return
value for each database call. Methods such as do
and execute return a true value if successful, so
you can say:
$dbh->do($SQL) or die $dbh->errstr;
$sth->execute( ) or die $sth->errstr;
The do method returns the number of rows affected,
but in such a way that it always returns a true value if successful.
(If you're curious how to do this, see the Introduction to Chapter 1 for the gory details of how Perl decides
what's true and what's false.)If you're debugging error catching and error handling, you might want
to add the PrintError attribute to your database connection:
$dbh = DBI->connect($DSN, $user, $password,
{ RaiseError => 1, PrintError => 1 });
When problems occur on the handle, PrintError issues a warning before
RaiseError calls die. So even if you catch the
error with eval and it doesn't necessarily kill
your program, you still get to see the text of the error message.
14.11.4. See Also
The documentation with the DBI module from CPAN; http://dbi.perl.org; Programming
the Perl DBI; Recipe 14.12
