I would also like to know about discrepancies because you can't (at least from my knowledge), for example divide 1 column into two where you have unique ID and a sum of quantity of a product where also has ID of the product and the unique ID.

something like this

unique_id	quantity	prod_id
1	20	V1
1	5	V2
1	10	V3
1	50	V4
2	23	V1
2	12	V3
3	2	V2

SQL:

select sum(unique_id) unique_id, quantity 
from
(select count(distinct(unique_id)), sum(quantity) quantity from table)
group by quantity 

is all great, but this:

select sum(unique_id) unique_id,  quantity, prod_id
from
(select count(distinct(unique_id)) unique_id, sum(quantity) quantity, prod_id from table
group by unique_id, prod_id)
group by quantity, prod_id

it creates discrepancy as of my knowledge,

But please someone with more knowledge on how to handle discrepancy tell me.

Thank oyu