Currently using the following code to return the total of all values recorded in the custom field called 'Distance'
<?php
$meta_key = 'Distance';//set this to appropriate custom field meta key
$allkms=$wpdb->get_var($wpdb->prepare("SELECT sum(meta_value) FROM $wpdb->postmeta WHERE meta_key = %s", $meta_key));
echo '<p><img src="/wp-content/themes/voidy/img/Walking.png" style="margin:0px; padding: 0px;" align="middle" alt="Activity Icon" />'.$allkms . ' kms</p>';
?>
I want to expand this to sum only those values where the post also has a value of 'Walking' in the custom field called 'Activity'
Any help much appreciated