Question

I have two tables like these:

CREATE TABLE people (
    id INT NOT NULL,
    PRIMARY KEY (id)
)

CREATE TABLE pairs (
    person_a_id INT,
    person_b_id INT,
    FOREIGN KEY (person_a_id) REFERENCES people(id),
    FOREIGN KEY (person_b_id) REFERENCES people(id) 
)

I want to select pairs of people at random from the people table, and after selecting them I add the randomly select pair to the pairs table. person_a_id always refers to the person with the lower id of the pair (since the order of the pair is not relevant).

The thing is that I never want to select the same pair twice, so I need to check the pairs table before I return my randomly selected pair.

Is it possible to do this using just a single SQL query in a reasonably efficient and elegant manner?

(I’m doing this using the Java Persistence API, but hopefully I’ll be able to translate any answers into JPA code)

Answer 1

select a.id, b.id
from people1 a
inner join people1 b on a.id < b.id
where not exists (
    select *
    from pairs1 c
    where c.person_a_id = a.id
      and c.person_b_id = b.id)
order by a.id * rand()
limit 1;

Limit 1 returns just one pair if you are “drawing lots” one at a time. Otherwise, up the limit to however many pairs you need.

The above query assumes that you can get

1 - 2
2 - 7

and that the pairing 2 - 7 is valid since it doesn’t exist, even if 2 is featured again. If you only want a person to feature in only one pair ever, then

select a.id, b.id
from people1 a
inner join people1 b on a.id < b.id
where not exists (
    select *
    from pairs1 c
    where c.person_a_id in (a.id, b.id))
  and not exists (
    select *
    from pairs1 c
    where c.person_b_id in (a.id, b.id))
order by a.id * rand()
limit 1;

If multiple pairs are to be generated in one single query, AND the destination table is still empty, you could use this single query. Take note that LIMIT 6 returns only 3 pairs.

select min(a) a, min(b) b
from
(
    select
      case when mod(@p,2) = 1 then id end a,
      case when mod(@p,2) = 0 then id end b,
      @p:=@p+1 grp
    from (
        select id
        from (select @p:=1) p, people1
        order by rand()
        limit 6
    ) x
) y
group by floor(grp/2)