Mysql – How to know how many rows a Perl DBI query returns

dbimysqlperl

I'm trying to basically do a search through the database with Perl to tell if there is an item with a certain ID. This search can return no rows, but it can also return one.

I have the following code:

my $th = $dbh->prepare(qq{SELECT bi_exim_id FROM bounce_info WHERE bi_exim_id = '$exid'});
$th->execute();
if ($th->fetch()->[0] != $exid) {
        ...

Basically, this tries to see if the ID was returned and if it's not, continue with the script. But it is throwing a Null array reference error on the $th->fetch()->[0] thing.
How can i just simply check to see if it returned rows or now?

Best Solution

The DBD::mysql driver has a the rows() method that can return the count of the results:

$sth = $dbh->prepare( ... );
$sth->execute;
$rows = $sth->rows;

This is database-driver specific, so it might not work in other drivers, or it might work differently in other drivers.

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Mysql – How to output MySQL query results in CSV format

From Save MySQL query results into a text or CSV file:

SELECT order_id,product_name,qty
FROM orders
WHERE foo = 'bar'
INTO OUTFILE '/var/lib/mysql-files/orders.csv'
FIELDS TERMINATED BY ','
ENCLOSED BY '"'
LINES TERMINATED BY '\n';

Note: That syntax may need to be reordered to

SELECT order_id,product_name,qty
INTO OUTFILE '/var/lib/mysql-files/orders.csv'
FIELDS TERMINATED BY ','
ENCLOSED BY '"'
LINES TERMINATED BY '\n'
FROM orders
WHERE foo = 'bar';

in more recent versions of MySQL.

Using this command, columns names will not be exported.

Also note that /var/lib/mysql-files/orders.csv will be on the server that is running MySQL. The user that the MySQL process is running under must have permissions to write to the directory chosen, or the command will fail.

If you want to write output to your local machine from a remote server (especially a hosted or virtualize machine such as Heroku or Amazon RDS), this solution is not suitable.

R – How to check if more than one record has been returned from DBI query in Perl

my $sth = $dbh->prepare("SELECT col1, col2, col3 FROM tablename");
$sth->execute();
my $rows = $sth->fetchall_arrayref({});
if (@$rows == 0) {
    die "no rows returned";
} elsif (@$rows > 1) {
    die "too many rows returned";
}
my $row = $rows->[0];
print "col1 is $row->{col1}\n";

Best Solution

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Mysql – How to output MySQL query results in CSV format

R – How to check if more than one record has been returned from DBI query in Perl

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