HOW TO ACCESS OTHER JOIN ID ON LEFT JOIN SUBQUERY - mysql

Question

Hello i have this query and I need to access de artist_id inside subquery. but don't work.
SELECT g.name, SUM(l.month_start_value), GROUP_CONCAT(l.artist_id), GROUP_CONCAT(l.month_start_value)
FROM genre g
LEFT JOIN genre_artist ga ON g.id = ga.genre_id
LEFT JOIN artist a ON ga.artist_id = a.id
LEFT JOIN (SELECT artist_id,
(SELECT CAST(value as SIGNED)
FROM platform_information_artist
WHERE platform_information_id =
(SELECT id from platform_information WHERE platform = 'spotify' AND information = 'monthly_listeners')
and artist_id = a.id
AND DATE(date) >= DATE(NOW()) - INTERVAL 30 DAY
ORDER BY date ASC
LIMIT 1) as month_start_value
FROM platform_information_artist
GROUP BY platform_information_artist.artist_id) l ON a.id = l.artist_id
GROUP BY g.id
ORDER BY g.id ASC;
The line that's wrong is
and artist_id = a.id
Returns this error: [42S22][1054] Unknown column 'a.id' in 'where clause'
I don't know why I can't access to a.id inside subquery.

Answer 1

There's a number of ways to do this. The least change from your current query is to simply change a couple of references:
SELECT g.name AS genre,
SUM(l.month_start_value) AS SumStartValue,
GROUP_CONCAT(l.artist_id) AS ArtistIDs,
GROUP_CONCAT(l.month_start_value) AS MonthStartValues
FROM genre g
LEFT JOIN genre_artist AS ga
ON g.id = ga.genre_id
LEFT JOIN artist AS a
ON ga.artist_id = a.id
LEFT JOIN
( SELECT pia.artist_id,
( SELECT CAST(pia2.value as SIGNED)
FROM platform_information_artist AS pia2
WHERE pia2.platform_information_id = (SELECT id
FROM platform_information
WHERE platform = 'spotify'
AND information = 'monthly_listeners')
AND pia2.artist_id = pia.artist_id
-- ^^ Updated from `a.id` to `pia.artist_id`
AND DATE(pia2.date) >= DATE(NOW()) - INTERVAL 30 DAY
ORDER BY pia2.date ASC
LIMIT 1
) as month_start_value
FROM platform_information_artist AS pia
GROUP BY pia.artist_id
) l
ON a.id = l.artist_id
GROUP BY g.id
ORDER BY g.id ASC;
This version should work in most versions of MySQL.
If you are using newer versions of MySql though you have a couple more options, which are both more efficient than the above. The first is to use a lateral join:
SELECT g.name AS genre,
SUM(l.month_start_value) AS SumStartValue,
GROUP_CONCAT(a.id) AS ArtistIDs,
GROUP_CONCAT(l.month_start_value) AS MonthStartValues
FROM genre g
LEFT JOIN genre_artist AS ga
ON g.id = ga.genre_id
LEFT JOIN artist AS a
ON ga.artist_id = a.id
LEFT JOIN LATERAL
( SELECT CAST(pia.value as SIGNED) AS month_start_value
FROM platform_information_artist AS pia
INNER JOIN platform_information AS i
ON i.id = pia.platform_information_id
WHERE i.platform = 'spotify'
AND i.information = 'monthly_listeners'
AND pia.artist_id = a.id
AND DATE(pia.date) >= DATE(NOW()) - INTERVAL 30 DAY
ORDER BY pia.date ASC
LIMIT 1
) AS l
ON 1= 1
GROUP BY g.id
ORDER BY g.id ASC;
The other is to use ROW_NUMBER() rather than limit to select one value per artist:
SELECT g.name AS genre,
SUM(l.month_start_value) AS SumStartValue,
GROUP_CONCAT(l.artist_id) AS ArtistIDs,
GROUP_CONCAT(l.month_start_value) AS MonthStartValues
FROM genre g
LEFT JOIN genre_artist AS ga
ON g.id = ga.genre_id
LEFT JOIN artist AS a
ON ga.artist_id = a.id
LEFT JOIN
( SELECT pia.artist_id,
CAST(pia.value as SIGNED) AS month_start_value,
ROW_NUMBER() OVER(PARTITION BY pia.artist_id ORDER BY pia.Date) AS RowNum
FROM platform_information_artist AS pia
INNER JOIN platform_information AS i
ON i.id = pia.platform_information_id
WHERE i.platform = 'spotify'
AND i.information = 'monthly_listeners'
AND DATE(pia.date) >= DATE(NOW()) - INTERVAL 30 DAY
) AS l
ON l.artist_id = a.id
AND l.RowNum = 1
GROUP BY g.id
ORDER BY g.id ASC;
Examples of all 3 on db<>fiddle