mysql> select * from test;
+----+----------+-------+-----------+
| id | name | score | subject |
+----+----------+-------+-----------+
| 1 | xiaoming | 89 | shuxue |
| 2 | xiaohong | 89 | shuxue |
| 3 | xiaohong | 80 | english |
| 4 | xiaohong | 80 | physics |
| 5 | xiaohong | 80 | astronaut |
| 6 | xiaoming | 80 | physics |
| 7 | xiaoming | 80 | astronaut |
| 8 | xiaoming | 80 | english |
| 9 | xiaobai | 80 | astronaut |
| 10 | xiaobai | 80 | english |
| 11 | xiaobai | 80 | physics |
| 12 | xiaobai | 80 | shuxue |
| 13 | xiaohei | 80 | astronaut |
| 14 | xiaohei | 80 | shuxue |
| 15 | xiaohei | 80 | physics |
| 16 | xiaohei | 80 | english |
+----+----------+-------+-----------+
16 rows in set (0.00 sec)
mysql> select avg(score) as shuxueavg from test where subject="shuxue";
+-----------+
| shuxueavg |
+-----------+
| 84.5000 |
+-----------+
1 row in set (0.00 sec)
mysql> select sum(score)/count(*) as shuxueavg from test where subject="shuxue";
+-----------+
| shuxueavg |
+-----------+
| 84.5000 |
+-----------+
1 row in set (0.00 sec)
