issue with joins - mysql

Question

I have the following query, in which I used JOINs. It says:
unknown column m.bv ..
Could you please take a look and tell me what I'm doing wrong?
$query4 = 'SELECT u.*, SUM(c.ts) AS total_sum1, SUM(m.bv) AS total_sum
FROM users u
LEFT JOIN
(SELECT user_id ,SUM(points) AS ts FROM coupon GROUP BY user_id) c
ON u.user_id=c.user_id
LEFT JOIN
(SELECT user_id ,SUM(points) AS bv FROM matching GROUP BY user_id) r
ON u.user_id=m.user_id
where u.user_id="'.$_SESSION['user_name'].'"
GROUP BY u.user_id';

Answer 1

You are selecting SUM(points) AS bv from the table with the alias r, there is no tables with the alias m. So that it has to be r.bv instead like so:
SELECT
u.*,
SUM(c.ts) AS total_sum1,
SUM(r.bv) AS total_sum
FROM users u
LEFT JOIN
(
SELECT
user_id,
SUM(points) AS ts
FROM coupon
GROUP BY user_id
) c ON u.user_id=c.user_id
LEFT JOIN
(
SELECT
user_id,
SUM(points) AS bv
FROM matching
GROUP BY user_id
) r ON u.user_id = m.user_id
where u.user_id="'.$_SESSION['user_name'].'"
GROUP BY u.user_id

Answer 2

Replace m., with r. Look at second Join

Answer 3

You have aliased the derived table with r and you reference that table (twice) with m. Correct one or the other.
Since you group by user_id in the two subqueries and user_id is (I assume) the primary key of table user, you don't really need the final GROUP BY.
I would write it like this, if it was meant for all (many) users:
SELECT u.*, COALESCE(c.ts, 0) AS total_sum1, COALESCE(m.bv, 0) AS total_sum
FROM users u
LEFT JOIN
(SELECT user_id, SUM(points) AS ts FROM coupon GROUP BY user_id) c
ON u.user_id = c.user_id
LEFT JOIN
(SELECT user_id, SUM(points) AS bv FROM matching GROUP BY user_id) m
ON u.user_id = m.user_id
and like this in your (one user) case:
SELECT u.*, COALESCE(c.ts, 0) AS total_sum1, COALESCE(m.bv, 0) AS total_sum
FROM users u
LEFT JOIN
(SELECT SUM(points) AS ts FROM coupon
WHERE user_id = "'.$_SESSION['user_name'].'") c
ON TRUE
LEFT JOIN
(SELECT SUM(points) AS bv FROM matching
WHERE user_id = "'.$_SESSION['user_name'].'") m
ON TRUE
WHERE u.user_id = "'.$_SESSION['user_name'].'"
The last query can also be simplified to:
SELECT u.*,
COALESCE( (SELECT SUM(points) FROM coupon
WHERE user_id = u.user_id)
, 0) AS total_sum1,
COALESCE( (SELECT SUM(points) FROM matching
WHERE user_id = u.user_id)
, 0) AS total_sum
FROM users u
WHERE u.user_id = "'.$_SESSION['user_name'].'"