mysql 空值计算，相加，处理

假如统计每科的总分数，出现了空值怎么办
用coalesce(sum(degreee),0)，例子如下：
课程表

mysql> select * from t_course;
+-------+-----------------+-----+
| cno   | cname           | tno |
+-------+-----------------+-----+
| 3-105 | 计算机导论      | 825 |
| 3-245 | 操作系统        | 804 |
| 6-166 | 数据电路        | 856 |
| 9-888 | 高等数学        | 100 |
+-------+-----------------+-----+

成绩表

mysql> select * from t_score;
+-----+-------+--------+
| sno | cno   | degree |
+-----+-------+--------+
| 103 | 3-245 |     86 |
| 105 | 3-245 |     75 |
| 109 | 3-245 |     68 |
| 103 | 3-105 |     92 |
| 105 | 3-105 |     88 |
| 109 | 3-105 |     76 |
| 101 | 3-105 |     64 |
| 107 | 3-105 |     91 |
| 108 | 3-105 |     78 |
| 101 | 6-166 |     85 |
| 107 | 6-106 |     79 |
| 108 | 6-166 |     81 |
+-----+-------+--------+

统计每个学科的总分数

select cname,coalesce(sum(b.degree),0) from t_course a left join t_score b on a.cno=b.cno group by b.cno;

mysql> select cname,coalesce(sum(b.degree),0) from t_course a left join t_score b on a.cno=b.cno group by b.cno;
+-----------------+---------------------------+
| cname           | coalesce(sum(b.degree),0) |
+-----------------+---------------------------+
| 高等数学        |                         0 |
| 计算机导论      |                       489 |
| 操作系统        |                       229 |
| 数据电路        |                       166 |
+-----------------+---------------------------+
4 rows in set (0.06 sec)