MySQL left joining subquery to group by - mysql

Question

I have a requirement where I need o group data into equal number ob rows. As mysql doesn't have rownum() I'm simulating this behaviour:
SET #row:=6;
SELECT MAX(agg.timestamp) AS timestamp, MAX(agg.value) AS value, COUNT(agg.value) AS count
FROM
(
SELECT timestamp, value, #row:=#row+1 AS row
FROM data
WHERE channel_id=52 AND timestamp >= 0 ORDER BY timestamp
) AS agg
GROUP BY row div 8
ORDER BY timestamp ASC;
Note: according to Can grouped expressions be used with variable assignments? this query may not be 100% correct, but it does work.
An additional requirement is to calculate the row difference between the grouped sets. I've looked for a solution joining the same table with a subquery:
SET #row:=6;
SELECT MAX(agg.timestamp) AS timestamp, MAX(agg.value) AS value, COUNT(agg.value) AS count
FROM
(
SELECT timestamp, value, #row:=#row+1 AS row
FROM data
WHERE channel_id=52 AND timestamp >= 0 ORDER BY timestamp
) AS agg
LEFT JOIN data AS prev
ON prev.channel_id = agg.channel_id
AND prev.timestamp = (
SELECT MAX(timestamp)
FROM data
WHERE data.channel_id = agg.channel_id
AND data.timestamp < MIN(agg.timestamp)
)
GROUP BY row div 8
ORDER BY timestamp ASC;
Unfortunately that errors:
Error Code: 1054. Unknown column 'agg.channel_id' in 'on clause'
Any idea how this query could be written?

Answer 1

You never selected channel_id from your sbuquery, so it's not returned to the parent query, and is therefore invisible. Try
SELECT MAX(agg.timestamp) AS timestamp, MAX(agg.value) AS value, COUNT(agg.value) AS count
FROM
(
SELECT timestamp, value, #row:=#row+1 AS row, channel_id
^^^^^^^^^^^^-- need this
FROM data
Since MySQL only sees and uses the fields you explicitly return from that subquery, and will NOT "dig deeper" into the table underlying the query, you need to select/return all of the fields you'll be using the parent queries.

Answer 2

How about this version:
SELECT MAX(agg.timestamp) AS timestamp, MAX(agg.value) AS value, COUNT(agg.value) AS count, COALESCE(prev.timestamp, 0) AS prev_timestamp
FROM (SELECT d.*, #row:=#row+1 AS row
FROM data d CROSS JOIN
(select #row := 6) vars
WHERE channel_id = 52 AND timestamp >= 0 ORDER BY timestamp
) agg LEFT JOIN
data prev
ON prev.channel_id = agg.channel_id AND
prev.timestamp = (SELECT MAX(timestamp)
FROM data
WHERE data.channel_id = agg.channel_id AND
data.timestamp < agg.timestamp
)
GROUP BY row div 8
ORDER BY timestamp ASC;
This includes all the columns in the subquery. And it puts the variable initialization in the same query.