按组的SQL值与最大列值的比率

Question

I thought this would be of general interest to the group because it is an actual job interview SQL question that I got wrong. I think I was close but can someone provide the correct answer?

Brand  |  Model  |  Price
Braun  |  KF7150 |  98
Braun  |  KF7000 |  70
Braun  |  KF400  |  55
Krups  |  KM730  |  67
Krups  |  KM4689 |  130
Krups  |  EC311  |  50

For the above table (Coffee), provide the code that displays brand, model, price, and the ratio of each model's price to the highest price of its brand, rounded to nearest 2 decimal places.

This code I know is wrong because it simply selects the overall max price (130).

SELECT Brand, Model, Price, ROUND(Price/(SELECT MAX(Price) FROM Coffee),2)
 as Price_to_Brand_Highest
FROM Coffee
ORDER BY Brand;

I tried a self join but all the ratios came back as 1 because each price was divided by itself.

SELECT C1.Brand, C1.Model, C1.Price, ROUND(C1.Price/(SELECT MAX(C2.Price) 
    FROM Coffee where C1.Brand=C2.Brand),2) as Price_to_Brand_Highest
FROM Coffee C1
JOIN Coffee C2 on C1.Model=C2.Model
GROUP BY C1.Model
ORDER BY C1.Brand;

Answer 1

You didn't specify a DBMS, so this is ANSI standard SQL:

select brand, model, price, 
       round(price / max(price) over (partition by brand), 2) as price_ratio
from coffee
order by brand, model;

Online example: http://rextester.com/HFZZRP41164

Answer 2

Before window functions were added to standard SQL, one method would be to join on a subquery that calculates the max value.

SELECT 
C1.*, 
ROUND(C1.Price/C2.MaxBrandPrice, 2) as Price_to_Brand_Highest
FROM Coffee C1
JOIN (
   Select Brand, max(Price) as MaxBrandPrice
   From Coffee
   Group by Brand
) C2
on (C1.Brand = C2.Brand)
ORDER BY C1.Brand, C1.Model;

Answer 3

; with Max_v as (
    Select Brand, max(Price) as MaxBrandPrice
        From Coffee
        Group by Brand
)
select Coffee.*, Max_v.price/(Max_ps.MaxBrandPrice*1.0)
    from Coffee, Max_v
    where D.Disease_ID = Max_ps.Disease_ID
    order by D.Disease_ID, D.ps desc