postgresql - How to select random trial with sql? (Bernoulli Trial) -

for each parent exists select 1 child row randomly. table structure looks this:

parent   id  child    parent_id    id    rank

where there 1..n child records each parent , rank unique per parent going 1 n.

the output should like:

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

with each parent producing 1 child row. (this serve basis bernoulli trials.)

(postgresql has random() function gives number between 0 , 1.)

not familiar @ postgresql, perhaps this?

select  distinct on (parent_id) parent_id "parent", id "child", rank    child c order         parent_id, random()

shell