Selection of rows related to a set of other rows.

Question

Let's say, I have two tables, "entities" and "tags". Relation between them is expressed by third table "entity_tags", which have tho fields: entity_id and tag_id. And I want to select all entities sorted by it's relevance to the given set of tags. For example, there is "banana", "orange", "potato", "pen" and "car" as entities and I want to filter all the entities that related to at least 2 of tags "fruit", "organic" and "item". What is the best way to do it?

First thing to come in mind is to use query like

select entity.*
     , count(*) as relevance 
  from entities
     , tags
     , entity_tags
 where entities.id=entity_tags.entity_id 
   and tags.id=entity_tags.tag_id
   and tags.name in ('fruit', 'organic', 'item')
group by entity.id 
order by relevance

but it isn't possible to add and relevance > 1 to where, sqlite is reporting about "misuse of aggregate". Result should look like this:

1|banana|3
2|orange|3
3|potato|2

And, by the way, is it possible to avoid nested select in counting of such rows?

select count(*) 
  from (<previous select>)

Answer 1

try this (untested)

select entity.*
     , count(*) as relevance 
  from entities
     , tags
     , entity_tags
 where entities.id=entity_tags.entity_id 
   and tags.id=entity_tags.tag_id
   and tags.name in ('fruit', 'organic', 'item')
group by entity.id  having count(entities.id) > 1
order by relevance

also look here for help on SQLite and search this page for "having"