SQL sum of custom field split into months

Update:

The following code works to an extent.
It outputs the sum of the custom field ‘total_invoice_cost per month ONLY if the custom field ‘paid’ has a value of ‘yes’. (which is what I wanted)

<?php
global $wpdb;
$results = $wpdb->get_results("SELECT SUM(x.total_invoice_cost),CONCAT( YEAR(x.post_date), '-', MONTH(x.post_date) ) AS grandtotal
FROM (
SELECT ID, total_invoice_cost.meta_value AS total_invoice_cost, paid.meta_value AS paid, wp_posts.post_date
FROM wp_posts
LEFT OUTER JOIN
(SELECT post_id, meta_value FROM wp_postmeta WHERE meta_key='paid') paid
ON wp_posts.id=paid.post_id
LEFT OUTER JOIN
(SELECT post_id, meta_value, meta_key FROM wp_postmeta WHERE meta_key='total_invoice_cost') total_invoice_cost
ON wp_posts.id=total_invoice_cost.post_id
) x
WHERE x.total_invoice_cost IS NOT NULL AND x.paid='yes' GROUP BY CONCAT( YEAR(x.post_date), '-', MONTH(x.post_date)
)");
print "<p>\n";
print_r($results);
print "</p>";

?>

However, it outputs the results like this:

Array ( [0] => stdClass Object ( [SUM(x.total_invoice_cost)] => 150 [grandtotal] => 2015-4 ) )

How can I edit this so that it shows it as £150 – 2015-4? I assume it has something to do with printing as a result, but I’m not sure how to change this.

If anyone needs to add multiple sums with different queries from the same table this is how you do it.

My example:
– I needed to show the sum of total_invoice_cost if paid had been set to ‘yes, no or overdue’.

– Each sum case has a different alias ie. yes, overdue, no, grandtotal

– These aliases are then used to individually show each sum (right at the bottom)

Here is the code:

<?php global $wpdb;
$results = $wpdb->get_results(
  	"SELECT SUM(total_invoice_cost) AS expecting,
	SUM(case WHEN paid='yes' THEN total_invoice_cost ELSE 0 END) AS yes,
	SUM(case WHEN paid='overdue' THEN total_invoice_cost ELSE 0 END) AS overdue,
	SUM(case WHEN paid='no' THEN total_invoice_cost ELSE 0 END) AS no,
	CONCAT( YEAR(post_date), '-', MONTH(post_date) ) AS grandtotal

	FROM (
	SELECT ID, total_invoice_cost.meta_value AS total_invoice_cost, paid.meta_value AS paid, wp_posts.post_date
	FROM wp_posts 

	LEFT OUTER JOIN
	(SELECT post_id, meta_value FROM wp_postmeta WHERE meta_key='paid') paid
	ON wp_posts.id=paid.post_id

	LEFT OUTER JOIN
	(SELECT post_id, meta_value, meta_key FROM wp_postmeta WHERE meta_key='total_invoice_cost') total_invoice_cost
	ON wp_posts.id=total_invoice_cost.post_id
	) x

	WHERE total_invoice_cost IS NOT NULL
	GROUP BY CONCAT( YEAR(post_date), '-', MONTH(post_date) )

");	

if(!empty($results)) {
     foreach($results as $r) {
	   	echo "<p>".$r->grandtotal."</p>";
	    echo "<p>EXPECTING IN THIS MONTH £".$r->expecting."</p>";
	   	echo "<p>PAID IN £".$r->yes."</p>";
	   	echo "<p>NOT PAID £".$r->no."</p>";
	    echo "<p>OVERDUE £".$r->overdue."</p>";

     }

} else {
     echo "<p>No invoices!</p>";
} 

  ?>

And here is an example output:

2014-3

EXPECTING IN THIS MONTH £50

PAID IN £50

NOT PAID £0

OVERDUE £0

2015-3

EXPECTING IN THIS MONTH £170

PAID IN £100

NOT PAID £70

OVERDUE £0