Oracle Specific Sorting - sql

Question

I have the following problem:
I need to sort some products where one needs to be a specific row and others to be random.
So if I have products: A B C D, I need for example B to be the third product while others can be random like:
C 1
A 2
B 3
D 4
Best shot I have tried is (3 is a dynamic value):
SELECT
product_name,
CASE
WHEN product = 'B' THEN 3
ELSE ( CASE WHEN rownum < 3 THEN rownum ELSE rownum + 1 END )
END sorting
FROM
products
ORDER BY
sorting ASC;
but I'm not always getting the desired outcome.
Any help or lead is appreciated.

Answer 1

This is rather tricky, but you can use row_number() and a bunch of arithmetic:
select p.*
from (select p.*,
row_number() over (order by case when product = 'B' then 2 else 1 end),
dbms_random.value
) as seqnum
from products p
) p
order by (case when seqnum < 3 then seqnum end),
(case when product = 'B' then 1 else 2 end),
seqnum;
The logic is:
Enumerate the values randomly, with the special value going last.
Put in the rows with lower values.
Put in the row with the special value.
Put in the rest of the rows.
The above uses a subquery because the randomness is enforced. You can do this without a subquery as:
order by (case when row_number() over (order by (case when product = 'B' then 2 else 1 end) < 3
then dbms_random.value
else 2 -- bigger than value
end),
(case when product = 'B' then 1 else 2 end),
dbms_random.value;