题目：
表:t_user

userid	class	score
1	a	90
1	b	80
1	c	70

编写SQL输出下列数据

userid	a	b	c
1	90	80	70

解题：

select 
  userid,
  max(case when class = 'a' then score end) as a,
  max(case when class = 'b' then score end) as b,
  max(case when class = 'c' then score end) as c  
from t_user
group by userid