DBI连库后的问题

DBI连库后的问题
chomp( my $aa = <STDIN>) ;
my $dbh = DBI -> connect("DBI:mysql:database=d1:host=test.com","user","password",);
my $sth = $dbh->prepare("SELECT uid,gid FROM table WHERE Name = '$aa'");
$sth->execute or die "can't execute the query: $sth->errstr";
my $ref = $sth ->fetchall_arrayref();

foreach my $ro (@$ref){

my $uid = $$ro[0];
my $gid = $$ro[1];
print"$uid\n";
}

请问如果我输入的$aa在table表中查不到的话,我想在foreach执行之前提示我输入错误，请问怎么做，谢谢了

谢谢了，我比较菜，谢谢.

$sth->execute有个返回.
$sth->execute有个返回值表示sql语句执行结果，这个结果表示语句结果的行数，判断这个结果就可以了

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