Random data sampling with oracle sql, data generation - sql

Question

i need to generate some sample data from a population. I want to do this with an SQL query on an Oracle 11g database.
Here is a simple working example with population size 4 and sample size 2:
with population as (
select 1 as val from dual union all
select 2 from dual union all
select 3 from dual union all
select 4 from dual)
select val from (
select val, dbms_random.value(0,10) AS RANDORDER
from population
order by randorder)
where rownum <= 2
(the oracle sample() funtion didn't work in connection with the WITH-clause for me)
But now I, I want to "upscale" or multiply my sample data. So that I can get something like 150 % sample data of the population data (population size 4 and sample size 6, e.g.)
Is there a good way to achieve this with an SQL query?

Answer 1

You could use CONNECT BY:
with population(val, RANDOMORDER) as (
select level, dbms_random.value(0,10) AS RANDORDER
from dual
connect by level <= 6
ORDER BY RANDORDER
)
select val
FROM population
WHERE rownum <= 4;
db<>fiddle demo

Answer 2

The solution depends, if you want all rows from first initial set(s) and random additional rows from last one then use:
with params(size_, sample_) as (select 4, 6 from dual)
select val
from (
select mod(level - 1, size_) + 1 val, sample_,
case when level <= size_ * floor(sample_ / size_) then 0
else dbms_random.value()
end rand
from params
connect by level <= size_ * ceil(sample_ / size_)
order by rand)
where rownum <= sample_
But if you allow possibility of result like (1, 1, 2, 2, 3, 3), where some values may not appear at all in output (here 4) then use this:
with params(size_, sample_) as (select 4, 6 from dual)
select val
from (
select mod(level - 1, size_) + 1 val, sample_, dbms_random.value() rand
from params
connect by level <= size_ * ceil(sample_ / size_)
order by rand)
where rownum <= sample_
How it works? We build set of (1, 2, 3, 4) as many times as it results from division sample / size. Then we assign random values. In first case I assign 0 to first set(s), so they will be in output for sure, and random values to last set. In second case randoms are assigned to all rows.