mysql基础 聚合函数 avg 求平均值

•        OS : Ubuntu 18.04.1 LTS
•        DBMS : mysql 8.0.12

示例

stu@Ubuntu:~$ mysql -u root -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 32
Server version: 8.0.12 MySQL Community Server - GPL

Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| employees          |
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
5 rows in set (0.00 sec)

mysql> use employees;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+----------------------+
| Tables_in_employees  |
+----------------------+
| current_dept_emp     |
| departments          |
| dept_emp             |
| dept_emp_latest_date |
| dept_manager         |
| employees            |
| salaries             |
| titles               |
+----------------------+
8 rows in set (0.01 sec)

mysql> describe salaries;
+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| emp_no    | int(11) | NO   | PRI | NULL    |       |
| salary    | int(11) | NO   |     | NULL    |       |
| from_date | date    | NO   | PRI | NULL    |       |
| to_date   | date    | NO   |     | NULL    |       |
+-----------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> select sum(salary) from salaries;
+--------------+
| sum(salary)  |
+--------------+
| 181480757419 |
+--------------+
1 row in set (0.62 sec)

mysql> select count(salary) from salaries;
+---------------+
| count(salary) |
+---------------+
|       2844047 |
+---------------+
1 row in set (0.41 sec)

mysql> select avg(salary) from salaries;
+-------------+
| avg(salary) |
+-------------+
|  63810.7448 |
+-------------+
1 row in set (0.65 sec)

mysql> exit
Bye
stu@Ubuntu:~$ 

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