MariaDB Avg()函数用于检索表达式的平均值。
语法：
SELECT AVG(aggregate_expression)
FROM tables
[WHERE conditions];
或者 –
SELECT expression1, expression2, ... expression_n,
AVG(aggregate_expression)
FROM tables
[WHERE conditions]
GROUP BY expression1, expression2, ... expression_n;
数据准备
在"testdb"数据库中创建一个"students"表，并插入一些数据。参考以下创建语句 –
USE testdb;
DROP TABLE students;
-- 创建新表
CREATE TABLE students(
student_id INT NOT NULL AUTO_INCREMENT,
student_name VARCHAR(100) NOT NULL,
student_address VARCHAR(40) default NULL,
admission_date DATE,
score float(4, 1) default NULL,
PRIMARY KEY ( student_id )
);
-- 插入数据
INSERT INTO students
(student_id, student_name, student_address,score, admission_date)
VALUES(1,'Maxsu','Haikou', 99.5,'2017-01-07 00:00:00');
INSERT INTO students
(student_id, student_name, student_address, score, admission_date)
VALUES
(2,'Crurry','Beijing',86,'2016-05-07 00:00:00'),
(3,'JMaster','Beijing',91,'2016-05-07 00:00:00'),
(4,'Mahesh','Guangzhou',78,'2016-06-07 00:00:00'),
(5,'Kobe','Shanghai',89,'2016-02-07 00:00:00'),
(6,'Blaba','Shengzhen',100,'2016-08-07 00:00:00');
1. AVG()函数与单表达式
示例：
查询Student表的平均分数。参考以下查询语句 –
SELECT AVG(Score) AS "Average Score" FROM students;
执行上面查询语句，得到以下结果 –
MariaDB [testdb]> SELECT AVG(Score) AS "Average Score" FROM students;
+----------------+
| Average Score |
+----------------+
| 90.58333 |
+----------------+
1 row in set (0.04 sec)
2. AVG()函数与公式
也可以在AVG()函数使用数学公式。 例如，如果要将学生的平均成绩提高50％，则可以使用以下公式来计算：
当前数据库中的记录如下 –
MariaDB [testdb]> select * from students;
+------------+--------------+-----------------+----------------+-------+
| student_id | student_name | student_address | admission_date | score |
+------------+--------------+-----------------+----------------+-------+
| 1 | Maxsu | Haikou | 2017-01-07 | 99.5 |
| 2 | Crurry | Beijing | 2016-05-07 | 86.0 |
| 3 | JMaster | Beijing | 2016-05-07 | 91.0 |
| 4 | Mahesh | Guangzhou | 2016-06-07 | 78.0 |
| 5 | Kobe | Shanghai | 2016-02-07 | 89.0 |
| 6 | Blaba | Shengzhen | 2016-08-07 | 100.0 |
+------------+--------------+-----------------+----------------+-------+
6 rows in set (0.07 sec)
示例：
SELECT AVG(score * 1.5) AS "New Score" FROM students;
执行上面查询语句，得到以下结果 –
MariaDB [testdb]> SELECT AVG(score * 1.5) AS "New Score" FROM students;
+-----------+
| New Score |
+-----------+
| 135.87500 |
+-----------+
1 row in set (0.01 sec)
2. AVG()函数与Order By子句
为了更容易说明问题，这里再插入一条记录 –
INSERT INTO students
(student_name, student_address,score, admission_date)
VALUES('Maxsu','Haikou', 90,'2017-11-17 00:00:00');
当前students表中，有以下数据 –
MariaDB [testdb]> select * from students;
+------------+--------------+-----------------+----------------+-------+
| student_id | student_name | student_address | admission_date | score |
+------------+--------------+-----------------+----------------+-------+
| 1 | Maxsu | Haikou | 2017-01-07 | 99.5 |
| 2 | Crurry | Beijing | 2016-05-07 | 86.0 |
| 3 | JMaster | Beijing | 2016-05-07 | 91.0 |
| 4 | Mahesh | Guangzhou | 2016-06-07 | 78.0 |
| 5 | Kobe | Shanghai | 2016-02-07 | 89.0 |
| 6 | Blaba | Shengzhen | 2016-08-07 | 100.0 |
| 7 | Maxsu | Haikou | 2017-11-17 | 90.0 |
+------------+--------------+-----------------+----------------+-------+
7 rows in set (0.00 sec)
参考下查询语句 –
SELECT student_name, AVG(score) AS "Average Salary"
FROM students
GROUP BY student_name;
执行上面查询语句，得到以下结果 –
MariaDB [testdb]> SELECT student_name, AVG(score) AS "Average Salary"
-> FROM students
-> GROUP BY student_name;
+--------------+----------------+
| student_name | Average Salary |
+--------------+----------------+
| Blaba | 100.00000 |
| Crurry | 86.00000 |
| JMaster | 91.00000 |
| Kobe | 89.00000 |
| Mahesh | 78.00000 |
| Maxsu | 94.75000 |
+--------------+----------------+
6 rows in set (0.02 sec)
