Generate random value from a list of values with even distribution in Oracle - sql

Question

I am trying to generate a random value from a list of valid values [1,2,3,4,5,9]. If I run my function 10 times, there are 2 values that never appear. I need every value in the list to be present in my sample. How can I ensure this ? An equal distribution would be good, but at least few rows of every value.
select random_code
from (
with temp_code_table as (
select '1' as random_code from dual union
select '2' as random_code from dual union
select '3' as random_code from dual union
select '4' as random_code from dual union
select '5' as random_code from dual union
select '9' as random_code from dual)
select random_code from temp_code_table order by dbms_random.value)
where rownum = 1;
I am running the above SQL inside a function, which checks that the random code generated is not the same as the original value.
Edit: Not sure the answer in the below post will help me achieve what I want.
Generating Random Value using CASE
Any advice on how to achieve this ?

Answer 1

"A random number" and "at least one of each" cannot be combined. The definition of random is that you down't know what you'll get... Roll a dice 10 times. Are you sure you'll have thrown all sides at least once ? No.
So to solve your problem, you can select one of each first - that way you have those already and then fill up the rest with randomly selected numbers from the set.
with temp_code_table (nr) as
(
SELECT '1' FROM dual UNION
SELECT '2' FROM dual UNION
SELECT '3' FROM dual UNION
SELECT '4' FROM dual UNION
SELECT '5' FROM dual UNION
SELECT '9' FROM dual
)
-- now select one of each. order doesn't matter, we're doing that at the end
, one_of_each AS
(
SELECT nr FROM temp_code_table
)
-- ok we got at least one of each. Now fill up the rest with randoms
-- generate a list of hundred random values 1,2,3,4,5,9 (100 is chosen randomly)
, hundred_random AS
(
SELECT CASE round(dbms_random.value(1,6))
WHEN 1 THEN '1'
WHEN 2 THEN '2'
WHEN 3 THEN '3'
WHEN 4 THEN '4'
WHEN 5 THEN '5'
WHEN 6 THEN '9'
END AS nr
FROM DUAL connect by LEVEL < 101
)
-- select 4 out of those.
, four_more (nr) AS
(
SELECT nr FROM hundred_random WHERE rownum < 5
)
-- put it all together
, ten_rows AS
(
SELECT nr FROM one_of_each
UNION ALL
SELECT nr FROM four_more
)
-- and shuffle...
SELECT nr FROM ten_rows order by dbms_random.value
;
The "temp_code_table" cte generates exactly 6 rows. That way you have at least one of each number in the set.
The "hundred_random" cte uses the case statement on the random number which will generate a single random value of the set. Then that is run 100 times using CONNECT BY LEVEL. Out of those 100, 4 are picked. Those 4 could all be 1 - this is random, there is no guarantee that you have distinct numbers.
At the end we union the 6 rows of the temp_code_table cte and the 4 rows of the four_more and order them randomly.