MySQL: Getting the results of these two subselects in one - mysql

Question

I searched a lot, but did not find a fitting answer, and after trying for several hours its time to ask someone who knows :)
Ok, This is the Query its about (all unneccessary content stripped):
SELECT
`id`,
`name`,
`details`,
`tour_type_id`,
`meeting_point_id`,
`start_date`,
`start_time`,
`end_date`,
`end_time`,
`max_guests`,
(SELECT
MAX(`level`)
FROM
`tour_notification`
WHERE
`model_id` = `tour`.`id`
AND `deactivated` != 1) `notification_level`,
(SELECT
MAX(`level`)
FROM
`guide_notification`
WHERE
(`tour1_id` = `tour`.`id`
OR `tour2_id` = `tour`.`id`)
AND `deactivated` != 1) `guide_notification_level`
FROM
`tour`
What I did not succeed in was to get the 2 MAX(level) in one:
Get the MAX from notification_level and guide_notification_level.
I do not need these values seperately, I just did not succeed getting in in one.
My best attempt was the following approach:
(SELECT
MAX(`level`)
FROM
(SELECT `level` FROM
`tour_notification`
WHERE
`model_id` = `tour`.`id`
AND `deactivated` != 1 UNION DISTINCT SELECT `level`
FROM
`guide_notification`
WHERE
(`tour1_id` = `tour`.`id`
OR `tour2_id` = `tour`.`id`)
AND `deactivated` != 1) as `notifications`) `notification_level`,
but here MySQL complained:
ERROR: Error 1054: Unknown column 'tour.id' in 'where clause'
If I add the tour table into the where clause of these subselects, it always gets the results for any items of tour table, not just the current one.
Anyone can push me in the right direction?
Thanks a lot!

Answer 1

The following might work for you. It provides three different ways of combining the levels into one column. Remove the ones you do not want.
SELECT
a.`id`,
a.`name`,
a.`details`,
a.`tour_type_id`,
a.`meeting_point_id`,
a.`start_date`,
a.`start_time`,
a.`end_date`,
a.`end_time`,
a.`max_guests`,
a.`tour_level`,
a.`guide_level`,
CONCAT(`tour_level`," ",`guide_level`) as `notification_level1`,
a.`tour_level` + a.`guide_level` as `notification_level2`,
GROUP_CONCAT(CONCAT(a.`tour_level`," ",a.`guide_level`) as `notification_level3`
FROM `tour` a
LEFT JOIN `tour_notification` b
ON a.`id` = b.`model_id` AND b.`deactivated` <> 1
LEFT JOIN `guide_notification` c
ON a.`id` = c.`tour2_id` AND c.`deactivated` <> 1
GROUP BY a.`id`

Answer 2

If you need the greatest value of two, you can use the GRATEST() function. But I think a UNION ALL subquery with LIMIT 1 would be more readable:
(
SELECT MAX(`level`) AS `max_level`
FROM `tour_notification`
WHERE `model_id` = `tour`.`id`
AND `deactivated` != 1
UNION ALL
SELECT MAX(`level`) AS `max_level`
FROM `guide_notification`
WHERE (`tour1_id` = `tour`.`id` OR `tour2_id` = `tour`.`id`)
AND `deactivated` != 1
ORDER BY max_level DESC
LIMIT 1
) AS `notification_level`