Question

I have 2 entities, say entity [x] and [y]. both entities are associated with some tags. entity [y] is used to search for similar [x] entities. For ex. if entity [y] consist of tags “books” and “blogs”, I want a list of all the [x] entities having both of those tags.

My table schema is:

[x] 
entity_id int    // its a foreign key from entity table  
tag varchar      // this is the tag to be matched
(one entity can have multiple tags hence multiple rows of same entity
id will be inserted)


[y]
entity_id int    // again a foreign key from entity2 table
tag varchar      // tags again
(same comment comes here too)

One more example, say I have a [y] entity_id 10, and this 10 entity_id is associated with 3 tags, say “bus”, “train” and “plane”, when I want to look up using this entity_id 10, I want a list of [x] entity_ids associated with all these 3 tags. [x] entity_id can also have additional tags too.

Answer 1

Given a y.entity_id as a parameter, 10 in this example, this query returns every x record that doesn’t have an instance where there is a y record that doesn’t have a corresponding x record:

SELECT
    *
FROM
    x
WHERE NOT EXISTS (
    SELECT *
    FROM 
        y LEFT JOIN
        x subX 
            ON subX.tag = y.tag
            AND subX.entity_id = x.entity_id
    WHERE 
        y.entity_id = 10
        AND subX.entity_id IS NULL  
)