SQL sum and add two columns from second query - php

Question

I'm trying to combine two bits of logic into one SQL query to speed up the effectiveness of my codebase, I currently have the two queries like so (pseudo code):
// Looping each user in table
$statement = "SELECT id FROM users";
// I then loop that statement, combining data from sub-query
{
// Get the "summing" data from table in reference to looped user
$second_statement = "SELECT add_col_1,add_col_2FROM users WHERE ref = id"
// Combine add_col_1, and add_col_2 together
array_sum($second_statement)
}
What I am after is the sum of that second statement, getting the ID from the first query SELECT id FROM users then researching that table for a reference to that, under the column name ref where I can then add together columns from each of those rows.
I'm not doing this so that I can get one statement instead of two, it's cause once the app scales, having to query thousands of users exponentially is just not good.
The statement I attempted to unite these two loops is:
SELECT
id,
(SUM(SELECT col_add_1+col_add_2 FROM users WHERE ref = a.id)) AS total
FROM users a
Got an execution error.

Answer 1

nearly there, you just need to join to itself to link up all the items in one call then group the results by id so the sum works on all the linked items.
SELECT
a.id,
SUM(b.col_add_1 + b.coll_add_2)
FROM
users a
LEFT JOIN
users b
ON
a.id = b.ref
GROUP BY
a.id

Answer 2

You have got an error because you are performing aggregation without involving any group by clause. So, you should re-write your query as
SELECT id,
(SELECT SUM(col_add_1+col_add_2) FROM users WHERE ref = a.id) AS total
FROM users a
GROUP BY id;