Question

Let’s say we have three tables in a relational database:

Person {id, name}
Obstacle {id, name}
Person_overcomes_obstacle {person_id, obstacle_id}

I want to write a query that tells me if at least one person has overcome all obstacles. Ideas?

Answer 1

I notice that I was the only person to use the natural table alias for Person_overcomes_obstacle!

You need a relational division query for this.

You can either count up the obstacles and the matching records in Person_overcomes_obstacle and return ones where the 2 numbers match or look at it another way as finding people for which there is no obstacle that they haven’t overcome.

SELECT p.id, p.name /*Or use COUNT(*) or wrap in Exists 
                     if you don't care about ids and names*/
FROM Person p
WHERE NOT EXISTS
   (SELECT * FROM Obstacle o
    WHERE NOT EXISTS 
    (
       SELECT * FROM Person_overcomes_obstacle poo
       WHERE poo.person_id = p.id and o.id = poo.obstacle_id
    )
)