Question

Im trying to make it select the group with the highest display priority.
The problem is that it selects the group with the lowest ID and I’m not sure how to select the group with the highest display priority.

The SQL query looks like this:

SELECT xf_user_group_relation.user_id , 
       xf_user_field_value.field_value,
       xf_user_group_relation.user_group_id, 
       MAX( xf_user_group.display_style_priority ) AS display_style_priority 
FROM xf_user_group_relation
     INNER JOIN xf_user_group ON xf_user_group_relation.user_group_id = xf_user_group.user_group_id
     INNER JOIN xf_user_field_value ON xf_user_group_relation.user_id = xf_user_field_value.user_id
WHERE xf_user_group.display_style_priority >= 500 
      AND field_id = 'minecraft' 
      AND xf_user_group_relation.user_group_id NOT IN(21,22)
GROUP BY xf_user_group_relation.user_id, 
      xf_user_field_value.field_value;

The groups are in xf_user_group with the structure:

user_group_id            int(10)        AUTO_INCREMENT
title                    varchar(50)
display_style_priority   int(10)
user_css                 text
user_title               varchar(100)

Answer 1

MySql errorneously lets you aggregate values without specifing ALL non-aggregates in GROUP BY. So it chooses on its own what value to show for user_group_id.
To get results you need, one would have to get maximum display_style_priority per user, and then join user_id and value back to original tables to filter them. There are two forms you might use, join:

SELECT *
FROM xf_user_group_relation
     INNER JOIN xf_user_group 
        ON xf_user_group_relation.user_group_id = xf_user_group.user_group_id
     INNER JOIN xf_user_field_value 
        ON xf_user_group_relation.user_id = xf_user_field_value.user_id
     INNER JOIN
     (
       SELECT r2.user_id, MAX(g2.display_style_priority) display_style_priority
         FROM xf_user_group_relation r2
        INNER JOIN xf_user_group g2 
           ON r2.user_group_id = g2.user_group_id
        GROUP BY r2.user_id
     ) maxDSP
        ON xf_user_group_relation.user_id = maxDSP.user_id
       AND xf_user_group.display_style_priority = maxDSP.display_style_priority
WHERE xf_user_group.display_style_priority >= 500 
      AND field_id = 'minecraft' 
      AND xf_user_group_relation.user_group_id NOT IN(21,22)

And compare to maximum in a subqery:

SELECT *
FROM xf_user_group_relation
     INNER JOIN xf_user_group 
        ON xf_user_group_relation.user_group_id = xf_user_group.user_group_id
     INNER JOIN xf_user_field_value 
        ON xf_user_group_relation.user_id = xf_user_field_value.user_id
WHERE xf_user_group.display_style_priority =
      (SELECT MAX(g2.display_style_priority) 
         FROM xf_user_group_relation r2
        INNER JOIN xf_user_group g2 
           ON r2.user_group_id = g2.user_group_id
        WHERE r2.user_id = xf_user_group_relation.user_id
      )
      AND xf_user_group.display_style_priority >= 500 
      AND field_id = 'minecraft' 
      AND xf_user_group_relation.user_group_id NOT IN(21,22)

Live demo is at Sql Fiddle.