php - mysql select relation two items

Solution:

I would do this with two subqueries andunion all:

select pid, max(tags), max(cats)
from ((select t.pid, group_concat(t.name) as tags, NULL as cats
       from tag_rel tr join
            tags t
            on tr.tid = .tid
       group by t.pid
      ) union all
      (select c.pid, NULL, group_concat(c.name) as cats
       from cat_rel cr join
            cats c
            on cr.cid = c.id
      )
     ) ct
group by pid;
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This will ensure that you get all posts, even those with no tags and/or no categories.