网站建设合作流程图,城乡互动联盟网站建设,网页基本三要素,北京房地产网站建设目录#xff1a; 前言一、日期函数获取日期获取时间获取时间戳在日期上增加时间在日期上减去时间计算两个日期相差多少天当前时间案例#xff1a;留言板 二、字符串函数查看字符串字符集字符串连接查找字符串大小写转换子串提取字符串长度字符串替换字符串比较消除左右空格案… 目录 前言一、日期函数获取日期获取时间获取时间戳在日期上增加时间在日期上减去时间计算两个日期相差多少天当前时间案例留言板 二、字符串函数查看字符串字符集字符串连接查找字符串大小写转换子串提取字符串长度字符串替换字符串比较消除左右空格案例 - 1姓名格式化案例 - 2学生成绩通知 三、数学函数绝对值进制转换取整规则格式化小数位随机数取模案例-1产生0 ~ 100随机数 四、其他函数查询当前用户查询当前正在使用的数据库数据加密ifnull条件判断 前言
剑指offer一年又7天 一、日期函数
函数名称描述current_date()当前日期年月日current_time()当前时间时分秒current_timestamp()当前时间戳年月日 时分秒date_add(date, interval num d_value_type)在date中添加日期或时间 d_value_type可选类型year、month、day、hour、minute、seconddate_sub(date, interval num d_value_type)在date中减去日期或时间datediff(date1, date2)两个日期的差date1 - date2单位是天now()当前日期时间 同current_timestamp()
案例
获取日期
mysql select current_date();
----------------
| current_date() |
----------------
| 2023-11-25 |
----------------
1 row in set (0.00 sec)获取时间
mysql select current_time();
----------------
| current_time() |
----------------
| 15:26:57 |
----------------
1 row in set (0.00 sec)获取时间戳
mysql select current_timestamp();
---------------------
| current_timestamp() |
---------------------
| 2023-11-25 15:27:17 |
---------------------
1 row in set (0.00 sec)在日期上增加时间
-- 当前日期
mysql select current_date();
----------------
| current_date() |
----------------
| 2023-11-25 |
----------------
1 row in set (0.01 sec)
-- 当前日期加10天
mysql select date_add(current_date(), interval 10 day);
-------------------------------------------
| date_add(current_date(), interval 10 day) |
-------------------------------------------
| 2023-12-05 |
-------------------------------------------
1 row in set (0.00 sec)
-- 指定日期加10天
mysql select date_add(2000-1-1, interval 10 day);
---------------------------------------
| date_add(2000-1-1, interval 10 day) |
---------------------------------------
| 2000-01-11 |
---------------------------------------
1 row in set (0.00 sec)
-- 当前时间加10分钟
mysql select date_add(now(), interval 10 minute);
-------------------------------------
| date_add(now(), interval 10 minute) |
-------------------------------------
| 2023-11-25 15:42:48 |
-------------------------------------
1 row in set (0.00 sec)
在日期上减去时间
-- 当前日期减10天
mysql select date_sub(current_date(), interval 10 day);
-------------------------------------------
| date_sub(current_date(), interval 10 day) |
-------------------------------------------
| 2023-11-15 |
-------------------------------------------
1 row in set (0.00 sec)计算两个日期相差多少天
-- date1 - date2
mysql select datediff(2023-11-24, 2023-11-25);
--------------------------------------
| datediff(2023-11-24, 2023-11-25) |
--------------------------------------
| -1 |
--------------------------------------
1 row in set (0.00 sec)mysql select datediff(2023-11-25, 2023-11-24);
--------------------------------------
| datediff(2023-11-25, 2023-11-24) |
--------------------------------------
| 1 |
--------------------------------------
1 row in set (0.00 sec)
-- 新中国成立至今天数
mysql select datediff(current_date(), 1949-10-1);
---------------------------------------
| datediff(current_date(), 1949-10-1) |
---------------------------------------
| 27083 |
---------------------------------------
1 row in set (0.00 sec)
-- 单位是天
mysql select datediff(now(), 1949-10-1 15:0:0);
-------------------------------------
| datediff(now(), 1949-10-1 15:0:0) |
-------------------------------------
| 27083 |
-------------------------------------
1 row in set (0.00 sec)当前时间
mysql select now();
---------------------
| now() |
---------------------
| 2023-11-25 15:46:03 |
---------------------
1 row in set (0.00 sec)
案例留言板
-- 创建留言板
mysql create table msg_tb(- id int unsigned primary key auto_increment,- name varchar(20) not null,- msg varchar(100) comment 留言信息,- msg_time timestamp- );
Query OK, 0 rows affected (0.02 sec)
-- 插入数据
mysql insert into msg_tb(name, msg) values(杜甫, 会当临绝顶);
Query OK, 1 row affected (0.01 sec)mysql insert into msg_tb(name, msg) values(杜甫, 一览众山小);
Query OK, 1 row affected (0.00 sec)mysql select * from msg_tb;
--------------------------------------------------
| id | name | msg | msg_time |
--------------------------------------------------
| 1 | 杜甫 | 会当临绝顶 | 2023-11-25 15:59:08 |
| 2 | 杜甫 | 一览众山小 | 2023-11-25 15:59:18 |
--------------------------------------------------
2 rows in set (0.00 sec)
要求查找两分钟以内的留言信息
-- 查找一次
mysql select name, msg from msg_tb where date_add(msg_time, interval 2 minute) now();
-------------------------
| name | msg |
-------------------------
| 杜甫 | 会当临绝顶 |
| 杜甫 | 一览众山小 |
-------------------------
2 rows in set (0.00 sec)
-- 再查找一次
mysql select name, msg from msg_tb where date_add(msg_time, interval 2 minute) now();
-------------------------
| name | msg |
-------------------------
| 杜甫 | 会当临绝顶 |
| 杜甫 | 一览众山小 |
-------------------------
2 rows in set (0.00 sec)
-- 两分钟之后
mysql select name, msg from msg_tb where date_add(msg_time, interval 2 minute) now();
Empty set (0.00 sec) 二、字符串函数
函数名称描述charset(str)返回字符串字符集concat(str1[, …])连接字符串instr(str, substr)返回substr在str中的位置位置从1开始没有返回0ucase(str) upper(str)转换成大写lcase(str) lower(str)转换成小写left(str, length) right(str, length)从str左/右边取length个字符length(str)str的长度replace(str, search_str, replace_str)在str中用replace_str替换search_strstrcmp(str1, str2)逐字符比较两字符串大小substring(str, pos[, length])从str的pos位置取length个字符默认取到结尾ltrim(str) rtrim(str) trim(str)去掉前空格后空格或者两边空格
查看字符串字符集
mysql select charset(aaa);
----------------
| charset(aaa) |
----------------
| utf8 | -- utf8编码
----------------
1 row in set (0.00 sec)mysql select charset(中国);
-------------------
| charset(中国) |
-------------------
| utf8 |
-------------------
1 row in set (0.00 sec)mysql select charset(123);
--------------
| charset(123) |
--------------
| binary | -- 二进制编码
--------------
1 row in set (0.00 sec)字符串连接
-- 两个字符串连接
mysql select concat(a, b);
------------------
| concat(a, b) |
------------------
| ab |
------------------
1 row in set (0.00 sec)
-- 多个字符串连接
mysql select concat(a, b, c);
-----------------------
| concat(a, b, c) |
-----------------------
| abc |
-----------------------
1 row in set (0.00 sec)
-- 数字转换为字符串进行拼接
mysql select concat(a, b, c, 1234);
-----------------------------
| concat(a, b, c, 1234) |
-----------------------------
| abc1234 |
-----------------------------
1 row in set (0.00 sec)
查找字符串
-- 位置从1开始
mysql select instr(abcd123efg, abc);
----------------------------
| instr(abcd123efg, abc) |
----------------------------
| 1 |
----------------------------
1 row in set (0.00 sec)
-- 不存在返回0
mysql select instr(abcd123efg, aaa);
----------------------------
| instr(abcd123efg, aaa) |
----------------------------
| 0 |
----------------------------
1 row in set (0.00 sec)
大小写转换
-- 字符串转大写
mysql select ucase(abcD);
---------------
| ucase(abcD) |
---------------
| ABCD |
---------------
1 row in set (0.00 sec)
-- 字符串转小写
mysql select lcase(ABCD);
---------------
| lcase(ABCD) |
---------------
| abcd |
---------------
1 row in set (0.00 sec)
-- 字符串转大写
mysql select upper(hello word);
---------------------
| upper(hello word) |
---------------------
| HELLO WORD |
---------------------
1 row in set (0.00 sec)
-- 字符串转小写
mysql select lower(ABCdefG);
------------------
| lower(ABCdefG) |
------------------
| abcdefg |
------------------
1 row in set (0.00 sec)
子串提取
-- 从左边开始提取3个字符
mysql select left(abcdefghhh3, 3);
------------------------
| left(abcdefghhh3, 3) |
------------------------
| abc |
------------------------
1 row in set (0.00 sec)
-- 从左边开始提取7个字符
mysql select left(abcdefghhh3, 7);
------------------------
| left(abcdefghhh3, 7) |
------------------------
| abcdefg |
------------------------
1 row in set (0.00 sec)
-- 从右边开始提取3个字符
mysql select right(abcdefghhh3, 3);
-------------------------
| right(abcdefghhh3, 3) |
-------------------------
| hh3 |
-------------------------
1 row in set (0.00 sec)
-- 从位置3开始提取到结尾
mysql select substring(abcdefghhh3, 3);
-----------------------------
| substring(abcdefghhh3, 3) |
-----------------------------
| cdefghhh3 |
-----------------------------
1 row in set (0.00 sec)
-- 从位置1开始提取到结尾
mysql select substring(abcdefghhh3, 1);
-----------------------------
| substring(abcdefghhh3, 1) |
-----------------------------
| abcdefghhh3 |
-----------------------------
1 row in set (0.00 sec)
-- 从位置1开始提取3个字符
mysql select substring(abcdefghhh3, 1, 3);
--------------------------------
| substring(abcdefghhh3, 1, 3) |
--------------------------------
| abc |
--------------------------------
1 row in set (0.00 sec)
字符串长度
mysql select length(abc);
---------------
| length(abc) |
---------------
| 3 |
---------------
1 row in set (0.00 sec)
-- 该数据库采用utf8编码utf8为变长编码集一个英文字母占一个字节一个汉字占三个字节
-- 注意字节和字符一个汉字是一个字符一个汉字占三个字节
-- length求的是字符串所占字节长度
mysql select length(中国);
------------------
| length(中国) |
------------------
| 6 |
------------------
1 row in set (0.00 sec)
-- 转换为字符串123求长度
mysql select length(123);
-------------
| length(123) |
-------------
| 3 |
-------------
1 row in set (0.00 sec)
字符串替换
mysql select replace(abc def abc, abc, hahaha) as replase;
-------------------
| replase |
-------------------
| hahaha def hahaha |
-------------------
1 row in set (0.00 sec)
-- 替换字符串不存在就不处理
mysql select replace(abc def abc, abcdef, hahaha) as replase;
-------------
| replase |
-------------
| abc def abc |
-------------
1 row in set (0.00 sec)
字符串比较
-- str1 str2
mysql select strcmp(abc, abc);
----------------------
| strcmp(abc, abc) |
----------------------
| 0 |
----------------------
1 row in set (0.00 sec)
-- str1 str2
mysql select strcmp(abc, aaaa);
-----------------------
| strcmp(abc, aaaa) |
-----------------------
| 1 |
-----------------------
1 row in set (0.00 sec)
-- str1 str2
mysql select strcmp(abc, b);
--------------------
| strcmp(abc, b) |
--------------------
| -1 |
--------------------
1 row in set (0.00 sec)
消除左右空格
-- 字符串
mysql select a bc as str;
-------------------------
| str |
-------------------------
| a bc |
-------------------------
1 row in set (0.00 sec)
-- 删去左边空格
mysql select ltrim( a bc ) as str;
---------------
| str |
---------------
| a bc |
---------------
1 row in set (0.00 sec)
-- 删去右边空格
mysql select rtrim( a bc ) as str;
------------------
| str |
------------------
| a bc |
------------------
1 row in set (0.00 sec)
-- 删去左右两边空格中间不处理
mysql select trim( a bc ) as str;
--------
| str |
--------
| a bc |
--------
1 row in set (0.00 sec)
案例 - 1姓名格式化
要求姓名首字母大写其他字母小写
-- 姓名数据
mysql select * from name_tb;
----------
| name |
----------
| lihua |
| XiaoMing |
| ZHANGWEI |
----------
3 rows in set (0.00 sec)
-- 首字母拆分
mysql select name, left(name, 1), substring(name, 1) from name_tb;
---------------------------------------------
| name | left(name, 1) | substring(name, 1) |
---------------------------------------------
| lihua | l | lihua |
| XiaoMing | X | XiaoMing |
| ZHANGWEI | Z | ZHANGWEI |
---------------------------------------------
3 rows in set (0.00 sec)
-- 大小写转换
mysql select name, ucase(left(name, 1)), lcase(substring(name, 2)) from name_tb;
-----------------------------------------------------------
| name | ucase(left(name, 1)) | lcase(substring(name, 2)) |
-----------------------------------------------------------
| lihua | L | ihua |
| XiaoMing | X | iaoming |
| ZHANGWEI | Z | hangwei |
-----------------------------------------------------------
3 rows in set (0.00 sec)
-- 拼接
mysql select name, concat(ucase(left(name, 1)), lcase(substring(name, 2))) as 姓名 from name_tb;
--------------------
| name | 姓名 |
--------------------
| lihua | Lihua |
| XiaoMing | Xiaoming |
| ZHANGWEI | Zhangwei |
--------------------
3 rows in set (0.00 sec)
案例 - 2学生成绩通知
格式XXX同学你好你本次考试总分XX语文XX数学XX英语XX。
-- 学生数据
mysql select * from grade;
-----------------------------------------------
| id | name | gander | chinese | math | english |
-----------------------------------------------
| 1 | 齐静春 | 男 | 134 | 98 | 56 |
| 2 | 陈平安 | 男 | 174 | 80 | 77 |
| 3 | 魏山君 | 男 | 176 | 98 | 90 |
| 5 | 刘羡阳 | 男 | 140 | 90 | 45 |
| 6 | 陈迹 | 男 | 140 | 95 | 30 |
| 7 | 郑大风 | 男 | 150 | 95 | 30 |
| 8 | 宁姚 | 女 | 99 | 99 | 99 |
| 9 | 陈暖树 | 女 | 90 | 89 | 80 |
-----------------------------------------------
8 rows in set (0.00 sec)
-- 先提取需要的信息
mysql select name, chinese math english as 总分, chinese, math, english from grade;
-------------------------------------------
| name | 总分 | chinese | math | english |
-------------------------------------------
| 齐静春 | 288 | 134 | 98 | 56 |
| 陈平安 | 331 | 174 | 80 | 77 |
| 魏山君 | 364 | 176 | 98 | 90 |
| 刘羡阳 | 275 | 140 | 90 | 45 |
| 陈迹 | 265 | 140 | 95 | 30 |
| 郑大风 | 275 | 150 | 95 | 30 |
| 宁姚 | 297 | 99 | 99 | 99 |
| 陈暖树 | 259 | 90 | 89 | 80 |
-------------------------------------------
8 rows in set (0.00 sec)
-- 使用 concat 函数拼接信息
mysql select concat(name, 同学你好, 你本次考试总分:, chinese math english, , 语文:, chinese, , 数学:, math, , 英语:, english, .) as 通知 - from grade;
-------------------------------------------------------------------------------------
| 通知 |
-------------------------------------------------------------------------------------
| 齐静春同学你好, 你本次考试总分:288, 语文:134, 数学:98, 英语:56. |
| 陈平安同学你好, 你本次考试总分:331, 语文:174, 数学:80, 英语:77. |
| 魏山君同学你好, 你本次考试总分:364, 语文:176, 数学:98, 英语:90. |
| 刘羡阳同学你好, 你本次考试总分:275, 语文:140, 数学:90, 英语:45. |
| 陈迹同学你好, 你本次考试总分:265, 语文:140, 数学:95, 英语:30. |
| 郑大风同学你好, 你本次考试总分:275, 语文:150, 数学:95, 英语:30. |
| 宁姚同学你好, 你本次考试总分:297, 语文:99, 数学:99, 英语:99. |
| 陈暖树同学你好, 你本次考试总分:259, 语文:90, 数学:89, 英语:80. |
-------------------------------------------------------------------------------------
8 rows in set (0.00 sec) 三、数学函数
函数名称描述abs(number)绝对值函数bin(decimal_number)十进制转二进制hex(decimal_number)十进制转十六进制conv(number, from_base, to_base)进制转换ceiling(number)向上取整floor(number)向下取整format(number, decimal_places)格式化保留小数位数rand()返回随机浮点数范围[0.0, 1.0)mod(number, denominator)取模求余
绝对值
mysql select abs(10);
---------
| abs(10) |
---------
| 10 |
---------
1 row in set (0.00 sec)mysql select abs(-10);
----------
| abs(-10) |
----------
| 10 |
----------
1 row in set (0.00 sec)mysql select abs(-10.01);
-------------
| abs(-10.01) |
-------------
| 10.01 |
-------------
1 row in set (0.04 sec)进制转换
-- 十进制到二进制
mysql select bin(2);
--------
| bin(2) |
--------
| 10 |
--------
1 row in set (0.00 sec)mysql select bin(4);
--------
| bin(4) |
--------
| 100 |
--------
1 row in set (0.00 sec)mysql select bin(15);
---------
| bin(15) |
---------
| 1111 |
---------
1 row in set (0.00 sec)
-- 十进制到十六进制
mysql select hex(15);
---------
| hex(15) |
---------
| F |
---------
1 row in set (0.00 sec)
-- 自定义进制转换十进制到二进制
mysql select conv(15, 10, 2);
-----------------
| conv(15, 10, 2) |
-----------------
| 1111 |
-----------------
1 row in set (0.00 sec)
-- 自定义进制转换二进制到十进制
mysql select conv(1111, 2, 10);
-------------------
| conv(1111, 2, 10) |
-------------------
| 15 |
-------------------
1 row in set (0.00 sec)
取整规则 -- 向上取整
mysql select ceiling(10.2);
---------------
| ceiling(10.2) |
---------------
| 11 |
---------------
1 row in set (0.00 sec)
-- 向上取整
mysql select ceiling(10.9);
---------------
| ceiling(10.9) |
---------------
| 11 |
---------------
1 row in set (0.00 sec)-- 向下取整
mysql select floor(10.2);
-------------
| floor(10.2) |
-------------
| 10 |
-------------
1 row in set (0.00 sec)
-- 向下取整
mysql select floor(10.9);
-------------
| floor(10.9) |
-------------
| 10 |
-------------
1 row in set (0.00 sec)-- 创建测试表
mysql create table int_tb(- num int- );
Query OK, 0 rows affected (0.02 sec)mysql insert into int_tb values(10), (10.2), (10.5), (10.9);
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0
-- 四舍五入
mysql select * from int_tb;
------
| num |
------
| 10 |
| 10 |
| 11 |
| 11 |
------
4 rows in set (0.00 sec)
// 向0取整比如C语言
int num1 10.9; // num1 实际等于10直接丢弃小数位
int num2 10.2; // num2 实际等于10格式化小数位
-- 保留两位小数
mysql select format(1.23456, 2);
--------------------
| format(1.23456, 2) |
--------------------
| 1.23 | -- 四舍五入
--------------------
1 row in set (0.00 sec)
-- 保留三位小数
mysql select format(1.23456, 3);
--------------------
| format(1.23456, 3) |
--------------------
| 1.235 | -- 四舍五入
--------------------
1 row in set (0.00 sec)
-- 保留十位小数
mysql select format(1.23456, 10);
---------------------
| format(1.23456, 10) |
---------------------
| 1.2345600000 |
---------------------
1 row in set (0.00 sec)
随机数
mysql select rand();
--------------------
| rand() |
--------------------
| 0.5852513821658225 |
--------------------
1 row in set (0.00 sec)mysql select rand();
---------------------
| rand() |
---------------------
| 0.09648454384550875 |
---------------------
1 row in set (0.00 sec)mysql select rand() * 100;
------------------
| rand() * 100 |
------------------
| 72.6668603463721 |
------------------
1 row in set (0.00 sec)取模
-- 101 % 10 10 ... 1
mysql select mod(101, 10);
--------------
| mod(101, 10) |
--------------
| 1 |
--------------
1 row in set (0.00 sec)
案例-1产生0 ~ 100随机数
要求0 ~ 100的整数
-- format 函数截取整数部分
mysql select format(rand() * 100, 0);
-------------------------
| format(rand() * 100, 0) |
-------------------------
| 34 |
-------------------------
1 row in set (0.00 sec)mysql select format(rand() * 100, 0);
-------------------------
| format(rand() * 100, 0) |
-------------------------
| 54 |
-------------------------
1 row in set (0.00 sec)
-- ceiling 向上取整
mysql select ceiling(rand() * 100);
-----------------------
| ceiling(rand() * 100) |
-----------------------
| 47 |
-----------------------
1 row in set (0.00 sec) 四、其他函数
函数名称描述user()查询当前用户datebase()显示当前正在使用的数据库md5(str)对一个字符串进行md5摘要摘要后得到一个32位字符串password(str)MySQL使用该函数对用户数据进行加密ifnull(val1, val2)如果val1为null返回val2否则返回val1
查询当前用户
mysql select user();
----------------
| user() |
----------------
| rootlocalhost |
----------------
1 row in set (0.00 sec)查询当前正在使用的数据库
mysql select database();
------------
| database() |
------------
| db2 |
------------
1 row in set (0.00 sec)
-- 使用数据库 db1
mysql use db1;
Database changedmysql select database();
------------
| database() |
------------
| db1 |
------------
1 row in set (0.00 sec)
数据加密
-- 创建操作表
mysql create table user_tb(- name varchar(20),- passwd varchar(32)- );
Query OK, 0 rows affected (0.03 sec)
-- 插入数据
mysql insert into user_tb(name, passwd) values(张三, 123456);
Query OK, 1 row affected (0.01 sec)mysql insert into user_tb(name, passwd) values(李四, 012345);
Query OK, 1 row affected (0.01 sec)
-- 如果对数据不做任何处理用户的密码明文保存如果公司数据库遭到攻击用户的信息就会被轻而易举的窃取
mysql select * from user_tb;
----------------
| name | passwd |
----------------
| 张三 | 123456 |
| 李四 | 012345 |
----------------
2 rows in set (0.00 sec)
-- md5() 加密
mysql update user_tb set passwd md5(123456) where name 张三;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0mysql select * from user_tb;
------------------------------------------
| name | passwd |
------------------------------------------
| 张三 | e10adc3949ba59abbe56e057f20f883e |
| 李四 | 012345 |
------------------------------------------
2 rows in set (0.00 sec)
-- 根据密码进行查找
mysql select name, passwd from user_tb where passwd 123456;
Empty set (0.00 sec)mysql select name, passwd from user_tb where passwd md5(123456);
------------------------------------------
| name | passwd |
------------------------------------------
| 张三 | e10adc3949ba59abbe56e057f20f883e |
------------------------------------------
1 row in set (0.00 sec)
-- password对密码要求的更加严格必须包含大小写字母数字以及特殊字符
mysql insert into user_tb values(王五, password(123456));
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements -- 密码不符合要求mysql insert into user_tb values(王五, password(123456wwDD));
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements -- 密码不符合要求mysql insert into user_tb values(王五, password(12345dwdAWDAWQ#$6));
Query OK, 1 row affected, 1 warning (0.01 sec)mysql insert into user_tb values(赵六, password(6666WWdd.));
Query OK, 1 row affected, 1 warning (0.01 sec)mysql select * from user_tb;
---------------------------------------------------
| name | passwd |
---------------------------------------------------
| 张三 | e10adc3949ba59abbe56e057f20f883e |
| 李四 | 012345 |
| 王五 | *67B40CCC0ED5939458DAF14EE1D77178C9615DFE |
| 赵六 | *94718C7C8D922CC41364D274CA13EEC71A67777B |
---------------------------------------------------
4 rows in set (0.00 sec) md5与password对比 md5和password都可以对数据进行加密 md5加密后生成32位字符串 password加密后生成41位字符串 md5对进行加密的字符串无要求 password要求进行加密的字符串必须包含大小写字母数字以及特殊字符 MySQL对用户信息加密时一般都使用password ifnull条件判断
-- str1 为null输出str2
mysql select ifnull(null, 123);
-------------------
| ifnull(null, 123) |
-------------------
| 123 |
-------------------
1 row in set (0.00 sec)
-- str1 为null输出str2
mysql select ifnull(null, null);
--------------------
| ifnull(null, null) |
--------------------
| NULL |
--------------------
1 row in set (0.00 sec)
-- str1 不为null输出str1
mysql select ifnull(666, 123);
------------------
| ifnull(666, 123) |
------------------
| 666 |
------------------
1 row in set (0.00 sec)类似于这样使用的C语言的三目操作符 exp1 ? exp1 : exp2;
