SUM two SQL rows with CodeIgniter in EE - php

Question

I'm working at summing two different rows and their field values (forum_total_topics and forum_total_posts) from a MySQL database in ExpressionEngine using the Active Record Class provided. I have tried multiple versions of code to sum the numbers. I have attempted and failed at passing a MySQL query to the request by saying.
$SQL = "SELECT forum_id, sum(forum_total_topics) + sum(forum_total_posts)
FROM exp_forums"
ee()->db->select($SQL);
$query = ee->db->get('exp_forums');
echo $query;
to echo the total sum, and getting the following error:
Error Number: 1064
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 2
SELECT (SELECT SUM(forum_total_topics) + SUM(forum_total_posts) FROM (exp_forums)
A messy solution
So I've tried splitting the entire request up to find a solution.
I finally got the follow piece of code working by returning multiple arrays and summing those, but it looks quite messy. How, can I return and sum the two rows from the table in one line or so?
$topics = ee()->db->select_sum('forum_total_topics')->get('exp_forums');
foreach($topics->result_array() as $topicrow) {
$totalTopics = $topicrow['forum_total_topics'];
}
$posts = ee()->db->select_sum('forum_total_posts')->get('exp_forums');
foreach($posts->result_array() as $postrow) {
$totalPosts = $postrow['forum_total_posts'];
}
$total = $totalTopics + $totalPosts;
Any suggestions would be greatly appreciated!
Tried suggestions
I attempted suggestions like so,
$SQL = "SELECT forum_id, SUM(forum_total_topics + forum_total_posts) AS total
FROM exp_forums
GROUP BY forum_id";
$query = ee()->db->select($SQL);
echo $query;
With this error instead.
A PHP Error was encountered
Severity: 4096
Message: Object of class CI_DB_mysql_driver could not be converted to string
Filename: libraries/Functions.php(679) : eval()'d code
Line Number: 98

Answer 1

Try it like this instead
SELECT forum_id, sum(forum_total_topics+forum_total_posts)
FROM exp_forums
GROUP BY forum_id
sqlfiddle demo

Answer 2

The first thing that you tried does not work due to a small syntax error I believe...
It should be:
$SQL = "forum_id, sum(forum_total_topics) + sum(forum_total_posts) FROM exp_forums"
The initial SELECT is being added in your class function so it should not appear in your $SQL:
ee()->db->select($SQL);
That's why your error starts:
SELECT (SELECT
This is also the case with the code under "Tried Suggestions".

Answer 3

Try this
SELECT forum_id, SUM(forum_total_topics + forum_total_posts) AS total
FROM exp_forums
GROUP BY forum_id