1.7 函数
1.7.1 数字类
-- 获取随机数
mysql> select rand();
+------------------+
| rand() |
+------------------+
| 0.25443412666622 |
+------------------+
1 row in set (0.00 sec)
-- 随机排序
mysql> select * from stuinfo order by rand();
-- 随机获取一条记录
mysql> select * from stuinfo order by rand() limit 1;
-- 四舍五入，向上取整，向下取整
mysql> select round(3.1415926,3) '四舍五入',truncate(3.14159,3) '截取数据',ceil(3.1) '向上取整',floor(3.9) '向下取整';
+----------+----------+----------+----------+
| 四舍五入 | 截取数据 | 向上取整 | 向下取整 |
+----------+----------+----------+----------+
| 3.142 | 3.141 | 4 | 3 |
+----------+----------+----------+----------+
1 row in set (0.04 sec)
注意： 截取数据直接截取，不四舍五入
1.7.2 字符串类
-- 大小写转换
mysql> select ucase('i name is tom') '转成大写',lcase('My Name IS TOM') '转成小写';
+---------------+----------------+
| 转成大写 | 转成小写 |
+---------------+----------------+
| I NAME IS TOM | my name is tom |
+---------------+----------------+
1 row in set (0.00 sec)
-- 截取字符串
mysql> select left('abcdef',3) '从左边截取',right('abcdef',3) '从右边截取',substring('abcdef',2,3) '字符串';
+------------+------------+--------+
| 从左边截取 | 从右边截取 | 字符串 |
+------------+------------+--------+
| abc | def | bcd |
+------------+------------+--------+
1 row in set (0.00 sec)
-- 字符串相连
mysql> select concat('中国','北京','顺义') '地址';
+--------------+
| 地址 |
+--------------+
| 中国北京顺义 |
+--------------+
1 row in set (0.00 sec)
mysql> select concat(stuname,'-',stusex) 信息 from stuinfo;
+-------------+
| 信息 |
+-------------+
| 张秋丽-男 |
| 李文才-男 |
| 李斯文-女 |
| 欧阳俊雄-男 |
| 诸葛丽丽-女 |
| 争青小子-男 |
| 梅超风-女 |
+-------------+
7 rows in set (0.00 sec)
-- coalesce(str1,str2) :str1有值显示str1,如果str1为空就显示str2
-- 将成绩为空的显示为缺考
mysql> select stuname,coalesce(writtenexam,'缺考'),coalesce(labexam,'缺考') from stuinfo natural left join stumarks;
+----------+------------------------------+--------------------------+
| stuname | coalesce(writtenexam,'缺考') | coalesce(labexam,'缺考') |
+----------+------------------------------+--------------------------+
| 张秋丽 | 77 | 82 |
| 李文才 | 50 | 90 |
| 李斯文 | 80 | 58 |
| 欧阳俊雄 | 65 | 50 |
| 诸葛丽丽 | 缺考 | 缺考 |
| 争青小子 | 56 | 48 |
| 梅超风 | 缺考 | 缺考 |
+----------+------------------------------+--------------------------+
7 rows in set (0.02 sec)
-- length（）:字节长度，char_length()：字符长度
mysql> select length('锄禾日当午') 字节,char_length('锄禾日当午') 字符;
+------+------+
| 字节 | 字符 |
+------+------+
| 10 | 5 |
+------+------+
1 row in set (0.00 sec)
1.7.3 时间类
-- 时间戳
mysql> select unix_timestamp();
+------------------+
| unix_timestamp() |
+------------------+
| 1560330458 |
+------------------+
1 row in set (0.00 sec)
-- 格式化时间戳
mysql> select from_unixtime(unix_timestamp());
+---------------------------------+
| from_unixtime(unix_timestamp()) |
+---------------------------------+
| 2019-06-12 17:08:18 |
+---------------------------------+
1 row in set (0.05 sec)
-- 获取当前格式化时间
mysql> select now();
+---------------------+
| now() |
+---------------------+
| 2019-06-12 17:08:50 |
+---------------------+
1 row in set (0.00 sec)
-- 获取年，月，日，小时，分钟，秒
mysql> select year(now()) 年,month(now()) 月,day(now()) 日,hour(now()) 小时,minute(now()) 分钟,second(now())秒;
+------+------+------+------+------+------+
| 年 | 月 | 日 | 小时 | 分钟 | 秒 |
+------+------+------+------+------+------+
| 2019 | 6 | 12 | 17 | 10 | 48 |
+------+------+------+------+------+------+
1 row in set (0.00 sec)
-- 星期，本年第几天;
mysql> select dayname(now()) 星期,dayofyear(now()) 本年第几天;
+-----------+------------+
| 星期 | 本年第几天 |
+-----------+------------+
| Wednesday | 163 |
+-----------+------------+
1 row in set (0.00 sec)
-- 日期相减
mysql> select datediff(now(),'2010-08-08') 相距天数;
+----------+
| 相距天数 |
+----------+
| 3230 |
+----------+
1 row in set (0.00 sec)
1.7.4 加密函数
1、md5()
2、sha()
mysql> select md5('aa');
+----------------------------------+
| md5('aa') |
+----------------------------------+
| 4124bc0a9335c27f086f24ba207a4912 |
+----------------------------------+
1 row in set (0.00 sec)
mysql> select sha('aa');
+------------------------------------------+
| sha('aa') |
+------------------------------------------+
| e0c9035898dd52fc65c41454cec9c4d2611bfb37 |
+------------------------------------------+
1 row in set (0.00 sec)
