多条件分类统计group by 显示数目为0的类别

Question

这是下面这个sql练习网站的第58个题目
http://www.sql-ex.ru/learn_exercises.php?LN=58
有一个表Product(maker, model, type)，
三种产品类别，想要统计每个生产商每个类别占每个生产商总产品个数的百分比。

maker model type
A 1232 PC
A 1233 PC
A 1276 Printer
A 1298 Laptop
A 1401 Printer
A 1408 Printer
A 1752 Laptop
B 1121 PC
B 1750 Laptop
C 1321 Laptop
D 1288 Printer
D 1433 Printer
E 1260 PC
E 1434 Printer
E 2112 PC
E 2113 PC

最终答案是
maker type prc
A Laptop 28.57
A PC 28.57
A Printer 42.86
B Laptop 50.00
B PC 50.00
B Printer .00
C Laptop 100.00
C PC .00
C Printer .00
D Laptop .00
D PC .00
D Printer 100.00
E Laptop .00
E PC 75.00
E Printer 25.00

c币没有多少了全拿出来，恳请各位路过的大神解答

Answer 1

目前的进度到了
WITH t AS(
SELECT maker, type, COUNT(model) as num
FROM Product
GROUP BY maker,type),
tt AS(
SELECT maker,SUM(num) as sum
FROM t
GROUP BY maker
)

SELECT t.maker, t.type, CAST( (t.num*100.0/tt.sum) AS decimal(5,2) ) as prc
FROM t JOIN tt ON t.maker = tt.maker

出来的结果是
maker type prc
A Laptop 28.57
A PC 28.57
A Printer 42.86
B Laptop 50.00
B PC 50.00
C Laptop 100.00
D Printer 100.00
E PC 75.00
E Printer 25.00

没有记录的生产商的类别没有0，不知道该怎么做