Populating relationship table randomly from two entity tables Postgresql - sql

Question

I'm attempting a simple E-R where the two entity sets are : Student & University each having about 200rows & the relationship to be "Applies" [so student applies to university]
I have the student table with a few data but the Primary Key being Student_ID
the University has Univeristy_Name as the PK;
now the relationship - "applies" - I have the student_ID & University_Name as FK & application_ID as PK
I have to populate this "applies" table containing about 10% chosen at random of the possible relationship, using one single INSERT statement & maybe a "random()" function. Does anyone know how do I go about populating the table using values from the other two tables randomly?

Answer 1

You would use:
insert into applies (student_id, university_name)
select s.student_id, u.university_name
from students s cross join
universities u
order by random()
limit 4000;
Alternatively, you can do this without sorting using:
insert into applies (student_id, university_name)
select s.student_id, u.university_name
from students s cross join
universities u
where random() < 0.1;
Note that this is an approximate 10% sample of the rows rather than an exact count.

Answer 2

You can cross join the two tables to get all combinations of student and university. Then number your rows in random order and keep those with a number <= the number of total rows divided by ten to keep 10% of those combinations:
insert into applies (student_id, university_name)
select student_id, university_name
from
(
select
s.student_id,
u.university_name,
row_number() over (order by random()) as rn,
count(*) over () as cnt
from student s
cross join university u
) randomized
where rn <= cnt / 10.0;

Answer 3

I replicated your case with
create table students (student_id serial primary key, name varchar);
create table university (University_Name varchar primary key);
insert into students (name) values ('Francesco');
insert into students (name) values ('Laura');
insert into students (name) values ('Christian');
insert into students (name) values ('Ugo');
insert into students (name) values ('Maria');
insert into students (name) values ('Antonietta');
insert into university values ('Bocconi');
insert into university values ('Universita di Pisa');
insert into university values ('Universita di Padova');
insert into university values ('Universita di Perugia');
If you don't have any limits in students applying for multiple universities, you could achieve the insert with
select * from students cross join university order by random() limit 3;
Where limit 3 displays only 3 rows