which group by clause must be added so the query returns the result shown?
A. group by ch.channel_type,t.month,rollup(co.country_code);
B. group by ch.channel_type,rollup(t.month,co.country_code);
C. group by CUBE(ch.channel_type,t.month,co.country_code);
D. group by ch.channel_type,t.month,co.country_code;
Answer：B
Explanation:
官方文档例题 Example 20-2 Partial ROLLUP 小计合并
提示：答案为个人理解和解答，难免有错，也欢迎大家及时指正。