Question

For each parent that exists I would like to select 1 child row randomly. The table structure looks like this:

parent
  id

child
   parent_id
   id
   rank

where there are 1..N child records for each parent and rank is unique per parent going from 1 to N.

The output should be like:

parent    child    rank
--------+-------+-------
 1        34       7
 2        56       8
...

With each parent producing one child row. (This will serve as a basis for Bernoulli Trials.)

(PostgreSQL has a random() function that gives a number between 0 and 1.)

Answer 1

not familiar at all with postgresql, but perhaps something like this?

SELECT  DISTINCT ON (parent_id) parent_id as "parent", id as "child", rank
FROM    child c
ORDER BY
        parent_id, random()