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MySQL学习笔记(四):内置函数和自定义函数

2017-05-30  本文已影响100人  maxwellyue

参考MySQL官方文档

一、常见内置函数

从文档列表可知,MySQL内置函数涵盖范围非常广:控制流函数、字符串函数、数值函数、时间日期函数、全文搜索函数 映射函数、XML函数、位函数、加密和压缩函数、信息函数、空间分析函数、Json函数、企业加密函数等等。在此只列举开发中常用的函数。

聚合函数

这些函数与其它函数有些不同:它们一般作用在多条记录上,一般与GROUP BY等关键字组合使用。

mysql> SELECT student_name, AVG(test_score)
    ->        FROM student
    ->        GROUP BY student_name;
mysql> SELECT student.student_name,COUNT(*)
    ->        FROM student,course
    ->        WHERE student.student_id=course.student_id
    ->        GROUP BY student_name;
#
# COUNT(*) is somewhat different in that it returns a count of the number of rows retrieved, 
# whether or not they contain NULL values.`
mysql> SELECT COUNT(DISTINCT results) FROM student;
mysql> SELECT student_name, MIN(test_score), MAX(test_score)
    ->        FROM student
    ->        GROUP BY student_name;
mysql> SELECT student_name, MIN(test_score), MAX(test_score)
    ->        FROM student
    ->        GROUP BY student_name;

字符串函数

mysql> SELECT CONCAT('My', 'S', 'QL');
        -> 'MySQL'
mysql> SELECT CONCAT('My', NULL, 'QL');
        -> NULL
mysql> SELECT CONCAT(14.3);
        -> '14.3'
mysql> SELECT INSERT('Quadratic', 3, 4, 'What');
        -> 'QuWhattic'
mysql> SELECT INSERT('Quadratic', -1, 4, 'What');
        -> 'Quadratic'
mysql> SELECT INSERT('Quadratic', 3, 100, 'What');
        -> 'QuWhat'
mysql> SELECT REPLACE('www.mysql.com', 'w', 'Ww');
        -> 'WwWwWw.mysql.com'
mysql> SELECT SUBSTRING('Quadratically',5);
        -> 'ratically'
mysql> SELECT SUBSTRING('foobarbar' FROM 4);
        -> 'barbar'
mysql> SELECT SUBSTRING('Quadratically',5,6);
        -> 'ratica'
mysql> SELECT SUBSTRING('Sakila', -3);
        -> 'ila'
mysql> SELECT SUBSTRING('Sakila', -5, 3);
        -> 'aki'
mysql> SELECT SUBSTRING('Sakila' FROM -4 FOR 2);
        -> 'ki'

数值函数

mysql> SELECT ABS(2);
        -> 2
mysql> SELECT ABS(-32);
        -> 32
mysql> SELECT CEILING(1.23);
        -> 2
mysql> SELECT CEILING(-1.23);
        -> -1
mysql> SELECT FLOOR(1.23), FLOOR(-1.23);
        -> 1, -2
mysql> SELECT MOD(234, 10);
        -> 4
mysql> SELECT 253 % 7;
        -> 1
mysql> SELECT MOD(29,9);
        -> 2
mysql> SELECT 29 MOD 9;
        -> 2
//要得到i <= R < j的随机数: FLOOR(i + RAND() * (j−i))
//比如,要得到5~10之间的随机数:FLOOR(5 + RAND()*5)

用法2:SELECT * FROM tbl_name ORDER BY RAND();:retrieve rows in random order

mysql> SELECT ROUND(-1.23);
        -> -1
mysql> SELECT ROUND(-1.58);
        -> -2
mysql> SELECT ROUND(1.58);
        -> 2
mysql> SELECT ROUND(1.298, 1);
        -> 1.3
mysql> SELECT ROUND(1.298, 0);
        -> 1
mysql> SELECT ROUND(23.298, -1);
        -> 20
mysql> SELECT TRUNCATE(1.223,1);
        -> 1.2
mysql> SELECT TRUNCATE(1.999,1);
        -> 1.9
mysql> SELECT TRUNCATE(1.999,0);
        -> 1
mysql> SELECT TRUNCATE(-1.999,1);
        -> -1.9
mysql> SELECT TRUNCATE(122,-2);
       -> 100
mysql> SELECT TRUNCATE(10.28*100,0);
       -> 1028

时间日期函数

mysql> SELECT CURDATE();
        -> '2008-06-13'
mysql> SELECT CURDATE() + 0;
        -> 20080613
mysql> SELECT CURTIME();
        -> '23:50:26'
mysql> SELECT CURTIME() + 0;
        -> 235026.000000
mysql> SELECT NOW();
        -> '2007-12-15 23:50:26'
mysql> SELECT NOW() + 0;
        -> 20071215235026.000000
mysql> SELECT UNIX_TIMESTAMP();
+------------------+
| UNIX_TIMESTAMP() |
+------------------+
|       1496132631 |
+------------------+
1 row in set
mysql> SELECT UNIX_TIMESTAMP('2017-05-30 16:25:19');
+---------------------------------------+
| UNIX_TIMESTAMP('2017-05-30 16:25:19') |
+---------------------------------------+
|                            1496132719 |
+---------------------------------------+
1 row in set
mysql> SELECT FROM_UNIXTIME(1447430881);
        -> '2015-11-13 10:08:01'
mysql> SELECT FROM_UNIXTIME(1447430881) + 0;
        -> 20151113100801
mysql> SELECT FROM_UNIXTIME(UNIX_TIMESTAMP(),
    ->                      '%Y %D %M %h:%i:%s %x');
        -> '2015 13th November 10:08:01 2015'
mysql> SELECT DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y');
        -> 'Sunday October 2009'
mysql> SELECT DATE_FORMAT('2007-10-04 22:23:00', '%H:%i:%s');
        -> '22:23:00'
mysql> SELECT DATE_FORMAT('1900-10-04 22:23:00', '%D %y %a %d %m %b %j');
        -> '4th 00 Thu 04 10 Oct 277'
mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00', '%H %k %I %r %T %S %w');
        -> '22 22 10 10:23:00 PM 22:23:00 00 6'
mysql> SELECT DATE_FORMAT('1999-01-01', '%X %V');
        -> '1998 52'
mysql> SELECT DATE_FORMAT('2006-06-00', '%d');
        -> '00'

其他函数

mysql> select database(), version(), user();
+------------+------------+----------------+
| database() | version()  | user()         |
+------------+------------+----------------+
| cpgl       | 5.7.17-log | root@localhost |
+------------+------------+----------------+
1 row in set

二、自定义函数

参考:MySql------自定义函数详解

基本语法

CREATE FUNCTION [db_name.]fn_name(func_parameter[,...])
RETURNS type
[characteristic...]
routine_body
#解释
db_name:数据库名,不指明则为当前数据库
type: 任何mysql支持的类型
routine_body: 函数体
characteristic:
LANGUAGE SQL | [NOT]DETERMINISTIC
| {CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA}
| SQL SECURITY {DEFINER | INVOKER}
| COMMENT 'String'

注意:函数中一定要有return返回值语句

DROP FUNCTION  [IF EXISTS]  fn_name;
ALTER FUNCTION fn_name [characteristic...]
SHOW FUNCTION STATUS [LIKE 'pattern']
SHOW CREATE FUNCTION fn_name;
mysql> set @a=1;
Query OK, 0 rows affected

c.变量语句必须写在复合语句开头,并且在其他语句的前面;
d.一次性可以声明多个变量;
DECLARE var_name[,...] type [DEFAULT value]

SELECT 表中某字段 INTO 变量名 FROM 表名 WHERE条件;

这种方式下,SELECT语句的查询结果只能有一个。

实例

例子1

#创建表
CREATE TABLE `t_user_main` (  
  `f_userId` int(10) NOT NULL AUTO_INCREMENT COMMENT '用户id,作为主键',  
  `f_userName` varchar(5) DEFAULT NULL COMMENT '用户名',  
  `f_age` int(3) DEFAULT NULL COMMENT '年龄',  
  PRIMARY KEY (`f_userId`)  
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;  
#插入数据
INSERT INTO t_user_main (f_userName, f_age)   
VALUES('one',24),('two',25),('three',26),('four',27),('five',28),('six',29);  

#创建一个函数  
DELIMITER $$ -- 定界符  
-- 开始创建函数  
CREATE FUNCTION user_main_fn(v_id INT)  
RETURNS VARCHAR(50)  
BEGIN  
  -- 定义变量  
  DECLARE v_userName VARCHAR(50);  
  -- 给定义的变量赋值  
  SELECT f_userName INTO v_userName FROM t_user_main   
  WHERE f_userId = v_id;  
  -- 返回函数处理结果  
  RETURN v_userName;  
END $$ -- 函数创建定界符  
DELIMITER ; 

例子2

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