MySQL如何查询exam表中选课门数前2名

Question

数据表是exam，内容如下：
s_id c_id score
1 1 86
1 2 80
1 3 79
1 4 89
1 5 76
2 1 69
2 2 84
2 3 83
2 4 69
2 5 65
3 1
3 2 57
3 4 82
3 5
4 1 95
4 2 69
4 3
4 4 82
4 5
5 1 95
5 3 72
5 4 58
5 5
6 1 77
6 2 71
6 3 71
6 4 71
7 1 95
7 2 93
7 3 87
7 4 101
7 5 89
8 1 79
8 2 83
8 3 82
8 4 50
8 5 56
9 2 71
9 3 55
9 4 70
10 1 81
10 2 73
10 3 80
10 4 79

Answer 1

选课门数？是指s_id的个数么，count一下就可以了
selcet s_id,count(s_id) c from exam group by s_id order by count(s_id) desc limit 2