Mysql Left OUTER JOIN with Subquery (wordpress)

Question

I am creating a query for MYSQL that will create a table from 4 tables.

SELECT xp.ID,
MAX((CASE WHEN (xum.meta_key = 'first_name') THEN xum.meta_value ELSE NULL END)) AS `first_name`,
MAX((CASE WHEN (xum.meta_key = 'last_name') THEN xum.meta_value ELSE NULL END)) AS `last_name`,
MAX((CASE WHEN (xum.meta_key = 'user_church') THEN xum.meta_value ELSE NULL END)) AS `church`,
MAX((CASE WHEN (xpm.meta_key = 'reg_user') THEN xpm.meta_value ELSE NULL END)) AS `user`,
MAX((CASE WHEN (xpm.meta_key = 'shirt_size') THEN xpm.meta_value ELSE NULL END)) AS `shirt_size`,
MAX((CASE WHEN (xpm.meta_key = 'reg_trip') THEN xpm.meta_value ELSE NULL END)) AS `trip_id`,
xp_2.post_title AS  'trip_name',
FROM xs_posts AS xp
LEFT OUTER JOIN xs_postmeta AS xpm ON xp.ID = xpm.post_id
LEFT OUTER JOIN xs_usermeta AS xum ON xum.user_id = (CASE WHEN (xpm.meta_key = 'reg_user') THEN xpm.meta_value ELSE NULL END )
LEFT OUTER JOIN xs_posts xp_2 ON xp_2.ID = (CASE WHEN (xpm.meta_key = 'reg_trip') THEN xpm.meta_value ELSE NULL END )
Where xp.post_type = 'trip_reg'
GROUP BY xp.ID

Which produces:

ID   - first_name - last_name- church - user - shirt_size - trip_id - trip_name
3025 - firstname -   lastname -   23  -   1  -    Large   -  2033  -    NULL

These tables are basic wordpress tables coming from. I use the wp_posts table 2 times.

wp_usermeta wp_posts wp_postmeta

The problem is that I can not get the trip name to populate based on the trip_id. IE the triop_id is a post id, and I am trying to get the post title from that. That custom post type is post_mission_trip.

If I add a Where clause at the end

AND xp_2.ID IS NOT NULL

I get this as the output:

ID   - first_name - last_name- church - user - shirt_size - trip_id - trip_name
3025 -   NULL     -   NULL    - NULL  - NULL  -   NULL   -   2033   -  Trip Name

Answer 1

Just wrap the xp_2.post_title in a MAX() aggregate.

  MAX(xp_2.post_title) AS `trip_name`
  ^^^^               ^

The problem is that the GROUP BY is collapsing the rows, and you're getting a value from an indeterminate row. That's an outer join, and the join predicate is equality on an expression that can return a NULL (the CASE expression)...

The MAX() aggregate will filter out the NULL values, and get a non-NULL value. The only way to get a NULL returned would be if all the rows had a NULL.)