How to compute average Cost in SQL based on category?

So let’s say we have a table called phone.

and i need another column to show the average cost of a brand phone
I know i can do something like this:
SELECT brand, AVG (cost)
FROM Phone
GROUP BY brand;

and get something like this table:

Can someone help me how i would get this result below using a select sql statement?

>Solution :

You can use avg(cost) over (partition) such as this:

select *, 
 round(avg(cost) over (partition by brand), 2) as avg_cost
from phone;