MySQL Database Design – How to Implement an Association with Restrictions

What you describe in your second paragraph sounds like the Entity Attribute Value pattern.

This is a known pattern for dealing with the type of data you are talking about. When you need to be able to be extensible, drive the types from the data and allow you to start tracking new types as you grow. It allows you stay flexible and agile and be a little more forgiving and generic in your database design and schema.

This pattern works, even in relational databases, but it does take more work to get good performance, and can cause you headaches down the line depending on how many rows, how many inserts you are doing and how frequently you query it. This SO Question talks about some of the pitfalls nicely in the firs answer listed with 20 votes. I have seen this model work well, especially in environments where the new types and entities really do come in and we want to let that be more application driven or user driven but we were fighting with performance more often in that model than a traditional relational database model but we needed the flexibility and we made it work. Definitely look at that SO question and do some more research on EAV and whatever DBMS you are using to find examples from others who have tried the same.

Is it safe to assume that for every relevant post the tag 88 occurs exactly once and the tag 5 also occurs exactly once?

Is there an index on post_id in table post_tags?

If the answer is yes to both, something like this might work:

select
      t1.id,
      t1.name,
      count (*) - 2 as kpl
from
              post_tags pt1
   inner join post_tags pt2 on (pt2.post_id = pt1.post_id)
   inner join post_tags pt3 on (pt3.post_id = pt1.post_id)
   inner join tags t1 on (t1.id = pt3.tag_id)
where
         pt1.tag_id = 88
    and  pt2.tag_id = 5
group by
   t1.id,
   t1.name
having
   count (*) > 12;