一、MySQL的算术表达式

　　算术表达式就是加减乘除的运算过程，主要是对一条数据中出现的数字进行统计和运算。

　　首先，有一张数据表，如下：

mysql> select * from test_score;
+----+-------+--------+-------+--------+--------+
| id | class | name   | yuwen | shuxue | yingyu |
+----+-------+--------+-------+--------+--------+
|  1 |     1 | 张三    |    95 |     90 |     97 |
|  2 |     1 | 李四    |    59 |     88 |     90 |
|  3 |     1 | 王五    |    99 |     99 |     99 |
|  4 |     1 | 马六    |    51 |     56 |     59 |
|  5 |     2 | 侯七    |    92 |     94 |     93 |
|  6 |     2 | 黑八    |    92 |     97 |     89 |
|  7 |     2 | 牛二    |    59 |     60 |     61 |
|  8 |     2 | 熊大    |    55 |     50 |     54 |
|  9 |     3 | 铁牛    |    53 |     54 |     51 |
| 10 |     3 | 铜鼓    |    49 |     47 |     54 |
| 11 |     3 | 钢巴    |    47 |     51 |     50 |
+----+-------+--------+-------+--------+--------+
11 rows in set (0.00 sec)

然后统计总分，如下：

#学生张三的成绩总分
mysql> SELECT (yuwen+shuxue+yingyu) AS total FROM test_score where name='张三';
+-------+
| total |
+-------+
|   282 |
+-------+
1 row in set (0.00 sec)

二、SUM()函数的使用

　　定义：SUM()函数用于计算一组值或表达式的总和。
　　以前上学时老师会统计每科的平均分，然后看看谁拉了后腿。先计算语文，就是先把班里语文得分相加得到总分，然后除以学生人数，得到的就是平均分，低于平均分的学生就要倒霉了。

#所有学生语文成绩的总分
mysql> SELECT sum(yuwen) AS yuwen_total FROM test_score;
+-------------+
| yuwen_total |
+-------------+
|         751 |
+-------------+
1 row in set (0.00 sec)

#学生人数
mysql> SELECT count(*) AS total FROM test_score;
+-------+
| total |
+-------+
|    11 |
+-------+
1 row in set (0.00 sec)

#求语文成绩的平均值
mysql> select sum(yuwen)/count(*) from test_score;
+---------------------+
| sum(yuwen)/count(*) |
+---------------------+
|             68.2727 |
+---------------------+
1 row in set (0.04 sec)

三、聚合函数

　　MySQL提供了许多聚合函数，包括AVG，COUNT，SUM，MIN，MAX等。除COUNT函数外，其它聚合函数在执行计算时会忽略NULL值。

#语文最高分
mysql> SELECT MAX(yuwen) FROM test_score;
+------------+
| MAX(yuwen) |
+------------+
|         99 |
+------------+
1 row in set (0.00 sec)

#语文最低分及所在班级
mysql> SELECT class,MIN(yuwen) FROM test_score;
+-------+------------+
| class | MIN(yuwen) |
+-------+------------+
|     1 |         47 |
+-------+------------+
1 row in set (0.00 sec)

#通过聚合函数求语文成绩的平均值
mysql> SELECT avg(yuwen) AS total FROM test_score;
+---------+
| total   |
+---------+
| 68.2727 |
+---------+
1 row in set (0.00 sec)

#格式化后语文成绩的平均值
mysql> SELECT format(avg(yuwen),2) AS total FROM test_score;
+-------+
| total |
+-------+
| 68.27 |
+-------+
1 row in set (0.00 sec)

#出去重复项后的语文成绩平均分
mysql> SELECT avg(DISTINCT yuwen) AS total FROM test_score;
+---------+
| total   |
+---------+
| 66.6667 |
+---------+
1 row in set (0.00 sec)

四、GROUP BY 与 HAVING 的应用

#GROUP BY分组
#各班级语文总分
mysql> SELECT class,sum(yuwen) AS yuwen_total   FROM test_score GROUP BY class;
+-------+-------------+
| class | yuwen_total |
+-------+-------------+
|     1 |         304 |
|     2 |         298 |
|     3 |         149 |
+-------+-------------+
3 rows in set (0.00 sec)

#各班级人数
mysql> SELECT class,count(*) AS total FROM test_score GROUP BY class;
+-------+-------+
| class | total |
+-------+-------+
|     1 |     4 |
|     2 |     4 |
|     3 |     3 |
+-------+-------+
3 rows in set (0.00 sec)

#各班级语文成绩的平均值
mysql> SELECT class,avg(yuwen) AS total FROM test_score GROUP BY class;
+-------+---------+
| class | total   |
+-------+---------+
|     1 | 76.0000 |
|     2 | 74.5000 |
|     3 | 49.6667 |
+-------+---------+
3 rows in set (0.00 sec)

#各班级语文最高分
mysql> SELECT class,MAX(yuwen) FROM test_score GROUP BY class;
+-------+------------+
| class | MAX(yuwen) |
+-------+------------+
|     1 |         99 |
|     2 |         92 |
|     3 |         53 |
+-------+------------+
3 rows in set (0.00 sec)

#语文最低分(并排序)
mysql> SELECT class,MIN(yuwen) AS yuwen FROM test_score GROUP BY class ORDER BY yuwen asc;
+-------+-------+
| class | yuwen |
+-------+-------+
|     3 |    47 |
|     1 |    51 |
|     2 |    55 |
+-------+-------+
3 rows in set (0.00 sec)

#各个班级的语文成绩最低分
mysql> SELECT class,MIN(yuwen) FROM test_score WHERE yuwen<60 GROUP BY class;
+-------+------------+
| class | MIN(yuwen) |
+-------+------------+
|     1 |         51 |
|     2 |         55 |
|     3 |         47 |
+-------+------------+
3 rows in set (0.00 sec)

#各个班级中平均分最低的班级
mysql> SELECT class,avg(yuwen) as yuwen FROM test_score GROUP BY class HAVING yuwen < 60;
+-------+---------+
| class | yuwen   |
+-------+---------+
|     3 | 49.6667 |
+-------+---------+
1 row in set (0.00 sec)