Question

I need to store a group of “tags” applied to objects. I was thinking of using something along the lines of :

But in order to implement gmail’s “Implicit Social Graph” algorithm (see this question), I need to be able to search for groups of tags that contain one or more specific tags.

So I guess my question is how to get the intersection of two groups of items, in mysql, in the most efficient way ?

Answer 1

find all sets containing one specific tag (given the value):

select tags_sets_id
from tags_has_sets, tags
where value = 'foo'
and tags_id = id;

find all sets containing either of two (or more) specific tags (given the values):

select distinct tags_sets_id 
from tags_has_sets, tags 
where value in ('foo', 'bar') 
and tags_id = id;

find all sets containing both of exactly two specific tags:

select t1.tags_sets_id 
from tags_has_sets t1, tags tags1, 
  tags_has_sets t2, tags tags2 
where tags1.value = 'foo' 
and tags2.value = 'bar' 
and t1.tags_id = tags1.id 
and t2.tags_id = tags2.id 
and t1.tags_sets_id = t2.tags_sets_id;

note that the last solution doesn’t generalize, but you could conceivably build a generalized algorithm to generate an n-joined sql statement on the fly.

here is one last implementation that does generalize, although i don’t know its performance characteristics compared to the generated-join way (thanks to @ypercube for an excellent enhancement to my initial suggestion):

select tags_sets_id
from tags_has_sets, tags
where value in ('foo', 'bar', 'baz')
and id = tags_id
group by tags_sets_id
having count(*) = 3;
-- formerly: having group_concat(distinct value order by value)
--   ='bar,baz,foo';