MySQL常用命令（七）-AVG、COUNT、SUM、MAX、MIN

汇总函数

函数	说明
AVG()	返回某列的平均值，自动忽略NULL值；
COUNT()	返回某列的行数；COUNT(*)包含NULL值所在的行，COUNT(column)计算特定的行，会忽略NULL。
MAX()	某列的最大值，忽略NULL值；可以用于日期和数值，不建议用于非日期和非数值；
MIN()	某列的最小值，忽略NULL值；
SUM()	返回某列之和，忽略NULL值；

mysql> SELECT * FROM person;
+----+-------+-----+----------+
| id | name  | age | salary   |
+----+-------+-----+----------+
|  1 | jack. |  23 |  8000.00 |
|  2 | Mary  |  21 |  8000.00 |
|  3 | Ken   |  25 | 10000.00 |
|  4 | jack. |  22 |  5600.00 |
+----+-------+-----+----------+
4 rows in set (0.00 sec)

mysql> update person set salary = null where name = 'jack';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0

mysql> SELECT * FROM person;
+----+-------+-----+----------+
| id | name  | age | salary   |
+----+-------+-----+----------+
|  1 | jack. |  23 |  8000.00 |
|  2 | Mary  |  21 |  8000.00 |
|  3 | Ken   |  25 | 10000.00 |
|  4 | jack. |  22 |  5600.00 |
+----+-------+-----+----------+
4 rows in set (0.00 sec)

mysql> SELECT AVG(salary) AS AVG_SALARY FROM person
    -> ;
+-------------+
| AVG_SALARY  |
+-------------+
| 7900.000000 |
+-------------+
1 row in set (0.00 sec)

mysql> SELECT MAX(salary) AS max_salary,MIN(salary) AS min_salary FROM person;
+------------+------------+
| max_salary | min_salary |
+------------+------------+
|   10000.00 |    5600.00 |
+------------+------------+
1 row in set (0.00 sec)

mysql> SELECT COUNT(*) AS count FROM person;
+-------+
| count |
+-------+
|     4 |
+-------+
1 row in set (0.00 sec)

mysql> SELECT SUM(salary) FROM person;
+-------------+
| SUM(salary) |
+-------------+
|    31600.00 |
+-------------+
1 row in set (0.00 sec)