数据表
假设有 student 表，数据如下
+----+-------+-------+----------+
| id | name | score | class_id |
+----+-------+-------+----------+
| 1 | 刘备1 | 90 | 1 |
| 2 | 刘备2 | 88 | 2 |
| 3 | 张飞1 | 70 | 1 |
| 4 | 张飞2 | 99 | 2 |
+----+-------+-------+----------+
问题 1：求每个班的最高分
解：select class_id, max(score) from student group by class_id;
+----------+------------+
| class_id | max(score) |
+----------+------------+
| 1 | 90 |
| 2 | 99 |
+----------+------------+
问题 2：求每个班的最低分
解：select class_id, min(score) from student group by class_id;
+----------+------------+
| class_id | min(score) |
+----------+------------+
| 1 | 70 |
| 2 | 88 |
+----------+------------+
问题 3：求每个班的平均分
解：select class_id, avg(score) from student group by class_id;
+----------+------------+
| class_id | avg(score) |
+----------+------------+
| 1 | 80.0000 |
| 2 | 93.5000 |
+----------+------------+
问题 4：求每个班的总分
解：select class_id, sum(score) from student group by class_id;
+----------+------------+
| class_id | sum(score) |
+----------+------------+
| 1 | 160 |
| 2 | 187 |
+----------+------------+