MySQL数据库函数

1. 数字类

函数	作用
rand()	生成随机数
order by rand()	随机排序
round(小数)	四舍五入
ceil(小数)	向上取整
floor(小数)	向下取整
truncate(小数,小数位数)	截取数字

例题：

mysql> select rand();           # 生成随机数
+---------------------+
| rand()              |
+---------------------+
| 0.18474003969201822 |
+---------------------+
1 row in set (0.00 sec)

mysql> select * from stuinfo order by rand();   # 随机排序

mysql> select * from stuinfo order by rand() limit 2;    # 随机抽两个学生
+--------+----------+--------+--------+---------+------------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress |
+--------+----------+--------+--------+---------+------------+
| s25305 | 诸葛丽丽         | 女      |     23 |       7 | 河南           |
| s25304 | 欧阳俊雄        | 男       |     28 |       4 | 天津           |
+--------+----------+--------+--------+---------+------------+
2 rows in set (0.00 sec)

mysql> select round(3.5);     #四舍五入
+------------+
| round(3.5) |
+------------+
|          4 |
+------------+
1 row in set (0.00 sec)

mysql> select ceil(3.1);    # 向上取整
+-----------+
| ceil(3.1) |
+-----------+
|         4 |
+-----------+
1 row in set (0.00 sec)

mysql> select floor(3.9);   # 向下取整
+------------+
| floor(3.9) |
+------------+
|          3 |
+------------+
1 row in set (0.00 sec)

mysql> select truncate(3.1415926,3);    # 截取数字
+-----------------------+
| truncate(3.1415926,3) |
+-----------------------+
|                 3.141 |
+-----------------------+
1 row in set (0.00 sec)

2. 字符串类

函数	作用
ucase(字符串)	转成大写
lcase(字符串)	转成小写
left(字符串, 个数)	从左边截取
right(字符串,个数)	从右边截取
substring(字符串, 开始位置,个数)	截取子字符串
concat(字符串,字符串...)	字符串相连
coalesce(字段1，字段2)	如果字段1不为空就显示字段1，否则，显示字段2
length(字符串)	获取字节长度
char_length	获取字符个数

例题：

mysql> select ucase('i am a boy!');     # 转成大写
+----------------------+
| ucase('i am a boy!') |
+----------------------+
| I AM A BOY!          |
+----------------------+
1 row in set (0.00 sec)

mysql> select lcase('I Am A Boy!');     #转成小写
+----------------------+
| lcase('I Am A Boy!') |
+----------------------+
| i am a boy!          |
+----------------------+
1 row in set (0.00 sec)

mysql> select left('abcde',3);      # 从左边开始截取，截取3个
+-----------------+
| left('abcde',3) |
+-----------------+
| abc             |
+-----------------+
1 row in set (0.00 sec)

mysql> select right('abcde',3);     # 从右边开始截取，截取3个
+------------------+
| right('abcde',3) |
+------------------+
| cde              |
+------------------+
1 row in set (0.00 sec)

mysql> select substring('abcde',2,3);   #从第2个位置开始截取，截取3个【位置从1开始】
+------------------------+
| substring('abcde',2,3) |
+------------------------+
| bcd                    |
+------------------------+
1 row in set (0.00 sec)

mysql> select concat('中国','上海');    # 字符串相连
+-----------------------+
| concat('中国','上海')       |
+-----------------------+
| 中国上海                    |
+-----------------------+
1 row in set (0.00 sec)

mysql> select concat(stuname,'-',stusex) from stuinfo;  # 将表中的姓名和性别连接起来
+----------------------------+
| concat(stuname,'-',stusex) |
+----------------------------+
| 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(字段1，字段2)  如果字段1不为空就显示字段1，否则，显示字段2
mysql> select stuname,coalesce(writtenexam,'缺考'),coalesce(labexam,'缺考') from stuinfo natural left join stumarks;   # 将考试成绩为空的显示为缺考
+----------+------------------------------+--------------------------+
| stuname  | coalesce(writtenexam,'缺考')    | coalesce(labexam,'缺考')    |
+----------+------------------------------+--------------------------+
| 张秋丽         | 77                           | 82                       |
| 李文才        | 50                           | 90                       |
| 李斯文        | 88                           | 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                        |
+----------+------------------------------+--------------------------+

mysql> select length('锄禾日当午');      # 字节长度
+----------------------+
| length('锄禾日当午')          |
+----------------------+
|                   10 |
+----------------------+
1 row in set (0.00 sec)

mysql> select char_length('锄禾日当午');     # 字符个数
+---------------------------+
| char_length('锄禾日当午')          |
+---------------------------+
|                         5 |
+---------------------------+
1 row in set (0.00 sec)

3. 时间类

函数	作用
unix_timestamp()	获取当前时间戳
unix_timestamp()	获取时间戳
from_unixtime(时间戳)	将时间戳转成年-月-日 小时:分钟:秒的格式
now()	获取当前日期时间
year(now())	获取当前年
month(now())	获取当前月
day(now())	获取当前日
hour(now())	获取当前时
minute(now())	获取当前分
second(now())	获取当前秒
dayname(now())	获取当前星期
monthname(now())	获取当前月（英文）
dayofyear(now())	获取本年的第几天
datediff(date1，date2)	日期相减
convert(now(),date/time)	将now()转成日期/时间
cast(now() as date/time)	将now()转成日期/时间

mysql> select unix_timestamp(); #获取时间戳
+------------------+
| unix_timestamp() |
+------------------+
|       1537084508 |
+------------------+
1 row in set (0.00 sec)

mysql> select from_unixtime(unix_timestamp());  # 将时间戳转成年-月-日 小时:分钟:秒的格式
+---------------------------------+
| from_unixtime(unix_timestamp()) |
+---------------------------------+
| 2018-09-16 15:55:56             |
+---------------------------------+
1 row in set (0.00 sec)

mysql> select now();        # 获取当前日期时间
+---------------------+
| now()               |
+---------------------+
| 2018-09-16 15:57:04 |
+---------------------+
1 row in set (0.00 sec)

mysql> select year(now()) 年,month(now()) 月, day(now()) 日,hour(now()) 小,minute(now()) 分钟,second(now()) 秒;
+------+------+------+------+------+------+
| 年     | 月     | 日     | 小时   | 分钟     | 秒     |
+------+------+------+------+------+------+
| 2018 |    9 |   16 |   15 |   59 |   14 |
+------+------+------+------+------+------+
1 row in set (0.00 sec)

mysql> select dayname(now()) 星期,monthname(now()),dayofyear(now()) 本年的第几天;
+--------+------------------+--------------+
| 星期       | monthname(now()) | 本年的第几天           |
+--------+------------------+--------------+
| Sunday | September        |          259 |
+--------+------------------+--------------+
1 row in set (0.00 sec)

mysql> select datediff(now(),'2008-8-8');   # 日期相减
+----------------------------+
| datediff(now(),'2008-8-8') |
+----------------------------+
|                       3691 |
+----------------------------+
1 row in set (0.00 sec)

mysql> select convert(now(),date),convert(now(),time);  # 将now()转成日期和时间
+---------------------+---------------------+
| convert(now(),date) | convert(now(),time) |
+---------------------+---------------------+
| 2018-09-16          | 16:07:24            |
+---------------------+---------------------+

mysql> select cast(now() as date),cast(now() as time);   # 将now()转成日期和时间
+---------------------+---------------------+
| cast(now() as date) | cast(now() as time) |
+---------------------+---------------------+
| 2018-09-16          | 16:08:03            |
+---------------------+---------------------+
1 row in set (0.00 sec)

4. 加密函数

函数	作用
md5(字符串)	md5加密
sha(字符创)	sha加密

+----------------------------------+------------------------------------------+
| md5('root')                      | sha('root')                              |
+----------------------------------+------------------------------------------+
| 63a9f0ea7bb98050796b649e85481845 | dc76e9f0c0006e8f919e0c515c66dbba3982f785 |
+----------------------------------+------------------------------------------+
1 row in set (0.00 sec)

5 判断函数

语法：

if(表达式,值1,值2)

例题：

mysql> select if(10%2=0,'偶数','奇数');
+--------------------------+
| if(10%2=0,'偶数','奇数')        |
+--------------------------+
| 偶数                        |
+--------------------------+
1 row in set (0.00 sec)

# 语文和数学都超过60分才通过
mysql> select stuname,ch,math,if(ch>=60 && math>=60,'通过','不通过') '是否通过' from stu;
+----------+------+------+----------+
| stuname  | ch   | math | 是否通过       |
+----------+------+------+----------+
| 张秋丽         |   80 | NULL | 不通过        |
| 李文才        |   77 |   76 | 通过        |
| 李斯文        |   55 |   82 | 不通过        |
| 欧阳俊雄        | NULL |   74 | 不通过        |
| 诸葛丽丽         |   72 |   56 | 不通过        |
| 争青小子        |   86 |   92 | 通过        |
| 梅超风        |   74 |   67 | 通过        |
| Tom      |   65 |   67 | 通过        |
| Tabm     |   88 |   77 | 通过        |
+----------+------+------+----------+
9 rows in set (0.00 sec)