wp-includes/meta.php LINE 765
if ( 'NOT EXISTS' == $meta_compare ) {
$join[$i] = "LEFT JOIN $meta_table";
$join[$i] .= $i ? " AS $alias" : '';
$join[$i] .= " ON ($primary_table.$primary_id_column = $alias.$meta_id_column AND $alias.meta_key = '$meta_key')";
$where[$k] = ' ' . $alias . '.' . $meta_id_column . ' IS NULL';
continue;
}