mysql 获取单个科目的平均分

mysql> select * from test;
+----+----------+-------+-----------+
| id | name     | score | subject   |
+----+----------+-------+-----------+
|   | xiaoming |     | shuxue    |
|   | xiaohong |     | shuxue    |
|   | xiaohong |     | english   |
|   | xiaohong |     | physics   |
|   | xiaohong |     | astronaut |
|   | xiaoming |     | physics   |
|   | xiaoming |     | astronaut |
|   | xiaoming |     | english   |
|   | xiaobai  |     | astronaut |
|  | xiaobai  |     | english   |
|  | xiaobai  |     | physics   |
|  | xiaobai  |     | shuxue    |
|  | xiaohei  |     | astronaut |
|  | xiaohei  |     | shuxue    |
|  | xiaohei  |     | physics   |
|  | xiaohei  |     | english   |
+----+----------+-------+-----------+
 rows in set (0.00 sec)

mysql> select avg(score) as shuxueavg from test where subject="shuxue";
+-----------+
| shuxueavg |
+-----------+
|   84.5000 |
+-----------+
 row in set (0.00 sec)

mysql> select sum(score)/count(*) as shuxueavg from test where subject="shuxue";
+-----------+
| shuxueavg |
+-----------+
|   84.5000 |
+-----------+
 row in set (0.00 sec)