SUM()

SUM()函数返回数值列的总数。

SUM 实例

查询课程表courses所有课程上课学生人数的总和

SELECT SUM(student_count) AS all_student_count
FROM courses;

执行输出结果

mysql> SELECT SUM(student_count) AS all_student_count
    -> FROM courses;
+-------------------+
| all_student_count |
+-------------------+
|              9290 |
+-------------------+
1 row in set (0.00 sec)

上述代码AS为给SUM(student_count)取别名

SUM 总结

经过上面实例我们总结出SUM的一般使用方法

语法

SELECT SUM(column_name) FROM table_name;

练习题：SUM

题目描述：统计课程表courses中教师teacher_id为3的课程 学生上课人数总和，结果用select_student_sum作为结果集列名

SELECT SUM(student_count) AS select_student_sum
FROM courses
WHERE teacher_id = 3;

目标输出结果

mysql> SELECT SUM(student_count) AS select_student_sum
    -> FROM courses
    -> WHERE teacher_id = 3;
+--------------------+
|select_student_sum |
+--------------------+
|               4570 |
+--------------------+
1 row in set (0.00 sec)