MySQL之函数

• 聚合函数
  统计指定列其值不为NULL的记录行数

SELECT COUNT (*) FROM 表名

计算指定列的最大值，如果指定列是字符串类型则使用字符串排序运算

SELECT MAX (字段名) FROM 表名;

计算指定列的最小值，如果指定列是字符串类型则使用字符串排序运算

SELECT MIN(字段名) FROM 表名;

计算指定列的数值和，如果指定列类型不是数值类型则计算结果为0

SELECT SUM (字段名）FROM 表名；

计算指定列的平均值，如果指定列类型不是数值类型则计算结果为0

SELECT AVG (字段名）FROM 表名;

例子：
创建一个表

CREATE TABLE student (
	sid CHAR(6),
	sname VARCHAR(50),
	age INT,
	gender VARCHAR(50) DEFAULT 'male'
);
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1001', 'lili', 14, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1002', 'wang', 15, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1003', 'tywd', 16, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1004', 'hfgs', 17, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1005', 'qwer', 18, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1006', 'zxsd', 19, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1007', 'hjop', 16, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1008', 'tyop', 15, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1009', 'nhmk', 13, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1010', 'xdfv', 17, 'female');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1012', 'lili', 14, 'male');
INSERT INTO student (sid,sname,age,gender) VALUES ('S_1013', 'wang', 15, 'female');

COUNT

mysql> SELECT COUNT(*) FROM student;
+----------+
| COUNT(*) |
+----------+
|       12 |
+----------+
1 row in set (0.05 sec)

mysql> SELECT COUNT(age) FROM student;
+------------+
| COUNT(age) |
+------------+
|         12 |
+------------+
1 row in set (0.04 sec)

MAX

mysql> SELECT MAX(age) FROM student;
+----------+
| MAX(age) |
+----------+
|       19 |
+----------+
1 row in set (0.08 sec)

MIN

mysql> SELECT MIN(age) FROM student;
+----------+
| MIN(age) |
+----------+
|       13 |
+----------+
1 row in set (0.05 sec)

SUM

mysql> SELECT SUM(age) FROM student;
+----------+
| SUM(age) |
+----------+
| 189      |
+----------+
1 row in set (0.05 sec)

AVG

mysql> SELECT AVG(age) FROM student;
+----------+
| AVG(age) |
+----------+
| 15.7500  |
+----------+
1 row in set (0.07 sec)

• 时间函数
• 字符串函数
• 数学函数
  跟编程语言中的输出没什么区别，这里就不进行描述。