MySQL学习笔记第七天

第07章单行函数

2. 数值函数

2.4 指数函数、对数函数

函数	用法
POW(x,y)，POWER(X,Y)	返回x的y次方
EXP(X)	返回e的x次方，其中e是一个常数，2.718281828459045
LN(X)，LOG(X)	返回以e为底的X的对数，当x<=0时，返回的结果为NULL
LOG10(X)	返回以10为底的X的对数，当X<=0时，返回的结果为NULL
LOG2(X)	返回以2为底的x的对数，当X<=0时，返回NULL

举例：

#指数
SELECT POW(2,5),POWER(2,4),EXP(2)
FROM DUAL;
#输出
+----------+------------+------------------+
| POW(2,5) | POWER(2,4) | EXP(2)           |
+----------+------------+------------------+
|       32 |         16 | 7.38905609893065 |
+----------+------------+------------------+
1 row in set (0.00 sec)#对数
SELECT LN(EXP(2)),LOG(EXP(2)),LOG10(10),LOG2(4)
FROM DUAL;
#输出
+------------+-------------+-----------+---------+
| LN(EXP(2)) | LOG(EXP(2)) | LOG10(10) | LOG2(4) |
+------------+-------------+-----------+---------+
|          2 |           2 |         1 |       2 |
+------------+-------------+-----------+---------+
1 row in set (0.00 sec)

2.5 进制间的转换

函数	用法
BIN(x)	返回x的二进制编码
HEX(x)	返回x的十六进制编码
OCT(x)	返回x的八进制编码
CONV(x,f1,f2)	返回f1进制数变成f2进制数

举例：

#进制间的转换
SELECT BIN(10),HEX(10),OCT(10),CONV(10,10,8)
FROM DUAL;
#输出
+---------+---------+---------+---------------+
| BIN(10) | HEX(10) | OCT(10) | CONV(10,10,8) |
+---------+---------+---------+---------------+
| 1010    | A       | 12      | 12            |
+---------+---------+---------+---------------+
1 row in set (0.01 sec)

3. 字符串函数

函数	用法
ASCIl(S)	返回字符串S中的第一个字符的ASCII码值
CHAR_LENGTH(s)	返回字符串s的字符数。作用与CHARACTER_LENGTH(s)相同
LENGTH(s)	返回字符串s的字节数，和字符集有关
CONCAT(s1,s2…sn)	连接s1,s2…,sn为一个字符串
CONCAT_WS(x,s1,s2,.sn)	同CONCAT(s1,s2,…sn)函数，但是每个字符串之间要加上x
INSERT(str, idx, len,replacestr)	将字符串str从第idx位置开始，len个字符长的子串替换为字符串replacestr
REPLACE(str, a, b)	用字符串b替换字符串str中所有出现的字符串a
UPPER(s)或UCASE(s)	将字符串s的所有字母转成大写字母
LOWER(s)或LCASE(s)	将字符串s的所有字母转成小写字母
LEFT(str,n)	返回字符串str最左边的n个字符
RIGHT(str,n)	返回字符串str最右边的n个字符
LPAD(str, len, pad)	用字符串pad对str最左边进行填充，直到str的长度为len个字符
RPAD(str ,len, pad)	用字符串pad对str最右边进行填充，直到str的长度为len个字符
LTRIM(s)	去掉字符串s左侧的空格
RTRIM(s)	去掉字符串s右侧的空格
TRIM(s)	去掉字符串s开始与结尾的空格
TRIM(s1 FROM s)	去掉字符串s开始与结尾的s1
TRIM(LEADING s1 FROM s)	去掉字符串s开始处的s1
TRIM(TRAILING s1 FROM s)	去掉字符串s结尾处的s1
REPEAT(str, n)	返回str重复n次的结果
SPACE(n)	返回n个空格
STRCMP(s1,s2)	比较字符串s1,s2的ASClI码值的大小
SUBSTR(s,index,len)	返回从字符串s的index位置其len个字符，作用与SUBSTRING(s,n,len)、MID(s,n,len)相同
LOCATE(substr,str)	返回字符串substr在字符串str中首次出现的位置，作用于POSITION(substr IN str)、INSTR(str,substr)相同。未找到返回0
ELT(m,1,s2,…n)	返回指定位置的字符串，如果m=1，则返回s1，如果m=2，则返回s2，如果m=n，则返回sn
FIELD(s,s1,s2…n)	返回字符串s在字符串列表中第一次出现的位置
FIND_IN_SET(s1,s2)	返回字符串s1在字符串s2中出现的位置。其中，字符串s2是一个以逗号分隔的字符串
REVERSE(s)	返回s反转后的字符串
NULLIF(value1,value2)	比较两个字符串，如果value1与value2相等，则返回NULL，否则返回value1

注意：MySQL中，字符串的位置是从1开始的。

举例：

#2. 字符串函数
#ASCII：求字符串中第一个字符的ASCII码
#CHAR_LENGTH:求字符串长度CHAR_LENGTH
#LENGTH:字符串所占字节数
SELECT ASCII('Abcdfsf'),CHAR_LENGTH('hello'),CHAR_LENGTH('我们')
FROM DUAL;
#输出
+------------------+----------------------+---------------------+
| ASCII('Abcdfsf') | CHAR_LENGTH('hello') | CHAR_LENGTH('我们') |
+------------------+----------------------+---------------------+
|               65 |                    5 |                   2 |
+------------------+----------------------+---------------------+
1 row in set (0.00 sec)SELECT LENGTH('hello'),LENGTH('我们')
FROM DUAL;
#一个汉字占3个字节
#输出
+-----------------+----------------+
| LENGTH('hello') | LENGTH('我们') |
+-----------------+----------------+
|               5 |              4 |
+-----------------+----------------+
1 row in set (0.00 sec)

#CONCAT：字符串拼接
# xxx worked for yyy
SELECT CONCAT(emp.last_name,' worked for ',mgr.last_name) "details"
FROM employees emp JOIN employees mgr
WHERE emp.`manager_id` = mgr.employee_id;

#CONCAT_WS:用第一个参数分隔连接后面的字符串
SELECT CONCAT_WS('-','hello','world','hello','beijing')
FROM DUAL;

#字符串的索引是从1开始的！(Java从0开始的)
#INSERT(str, idx, len,replacestr)
#将字符串str从第idx位置开始，len个字符长的子串替换为字符串replacestr
SELECT INSERT('helloworld',2,3,'aaaaa'),REPLACE('hello','lol','mmm'),REPLACE('hello','lo','mmm')
FROM DUAL;
#输出
+-------------------------------+------------------------------+---------------------------+
|INSERT('helloworld',2,3,'aaaaa')|REPLACE('hello','lol','mmm') | REPLACE('hello','lo','mmm') |
+-------------------------------+-----------------------------+-----------------------------+
| haaaaaoworld                  | hello                        | helmmm                      |
+--------------------------------+----------------------------+-----------------------------+
1 row in set (0.00 sec)#大小写转换
SELECT UPPER('HelLo'),LOWER('HelLo')
FROM DUAL;
#输出
+----------------+----------------+
| UPPER('HelLo') | LOWER('HelLo') |
+----------------+----------------+
| HELLO          | hello          |
+----------------+----------------+
1 row in set (0.00 sec)SELECT last_name,salary
FROM employees
WHERE LOWER(last_name) = 'King';
#严格说应该查不到-->但是Mysql大小写不严格
#输出
+-----------+----------+
| last_name | salary   |
+-----------+----------+
| King      | 24000.00 |
| King      | 10000.00 |
+-----------+----------+
2 rows in set (0.00 sec)SELECT LEFT('hello',2),RIGHT('hello',3),RIGHT('hello',13)
FROM DUAL;
#输出
+-----------------+------------------+-------------------+
| LEFT('hello',2) | RIGHT('hello',3) | RIGHT('hello',13) |
+-----------------+------------------+-------------------+
| he              | llo              | hello             |
+-----------------+------------------+-------------------+
1 row in set (0.00 sec)# LPAD:实现右对齐效果
# RPAD:实现左对齐效果
SELECT employee_id,last_name,LPAD(salary,10,'*'),LPAD(salary,10,' ')
FROM employees;

SELECT employee_id,last_name,RPAD(salary,10,'*'),RPAD(salary,10,' ')
FROM employees;

#TRIM去掉首尾空格
#LTRIM去掉左侧空格
#TRIM('oo' FROM 'ooheolloo')去掉'oo'
SELECT CONCAT('---',LTRIM('    h  el  lo   '),'***'),
TRIM('oo' FROM 'ooheolloo')
FROM DUAL;#REPEAT(str,n):重复n次str 
#SPACE(n):提供n个空格
#STRCMP：比较字符串大小
SELECT REPEAT('hello',4),LENGTH(SPACE(5)),STRCMP('abc','abe')
FROM DUAL;
#输出
+----------------------+------------------+---------------------+
| REPEAT('hello',4)    | LENGTH(SPACE(5)) | STRCMP('abc','abe') |
+----------------------+------------------+---------------------+
| hellohellohellohello |                5 |                  -1 |
+----------------------+------------------+---------------------+
1 row in set (0.00 sec)#SUBSTR(str,i,len):截取str中i处起len个字符
#LOCATE('ll','hello')定位‘ll’首次出现的位置
SELECT SUBSTR('hello',2,2),LOCATE('ll','hello'),LOCATE('lll','hello')
FROM DUAL;
#输出
+---------------------+----------------------+-----------------------+
| SUBSTR('hello',2,2) | LOCATE('ll','hello') | LOCATE('lll','hello') |
+---------------------+----------------------+-----------------------+
| el                  |                    3 |                     0 |
+---------------------+----------------------+-----------------------+
1 row in set (0.00 sec)#ELT:返回指定位置的字符串
#FIELD(s,s1,...)：返回s在字符列表中首次出现的位置
#FIND_IN_SET(s1,s2):返回s1在s2中首次出现的位置
SELECT ELT(2,'a','b','c','d'),FIELD('mm','gg','jj','mm','dd','mm'),
FIND_IN_SET('mm','gg,mm,jj,dd,mm,gg')
FROM DUAL;
#输出
+------------------------+--------------------------------------+---------------------------------------+
| ELT(2,'a','b','c','d') | FIELD('mm','gg','jj','mm','dd','mm') | FIND_IN_SET('mm','gg,mm,jj,dd,mm,gg') |
+------------------------+--------------------------------------+---------------------------------------+
| b                      |                                    3 |                                     2 |
+------------------------+--------------------------------------+---------------------------------------+
1 row in set (0.01 sec)

#NULLIF(s1,s2):字符串s1和s2相等返回NULL,不相等返回s1
SELECT employee_id,NULLIF(LENGTH(first_name),LENGTH(last_name)) "compare"
FROM employees;

4.日期和时间函数

4.1 获取日期、时间

函数	用法
CURDATE() ,CURRENT_DATE()	返回当前日期，只包含年、月、日
CURTIME(), CURRENT_TIME()	返回当前时间，只包含时、分、秒
NOW() / SYSDATE()/ CURRENT_TIMESTAMP() /LOCALTIME()/LOCALTIMESTAMP()	返回当前系统日期和时间
UTC_DATE()	返回UTC(世界标准时间)
UTC_TIME()	返回UTC(世界标准时间)时间

举例：

SELECT CURDATE(),CURRENT_DATE(),CURTIME(),CURRENT_TIME(),
NOW(),SYSDATE(),UTC_DATE(),UTC_TIME()
FROM DUAL;
#输出
+------------+----------------+-----------+----------------+---------------------+---------------------+------------+------------+
| CURDATE()  | CURRENT_DATE() | CURTIME() | CURRENT_TIME() | NOW()               | SYSDATE()           | UTC_DATE() | UTC_TIME() |
+------------+----------------+-----------+----------------+---------------------+---------------------+------------+------------+
| 2023-05-02 | 2023-05-02     | 21:42:09  | 21:42:09       | 2023-05-02 21:42:09 | 2023-05-02 21:42:09 | 2023-05-02 | 13:42:09   |
+------------+----------------+-----------+----------------+---------------------+---------------------+------------+------------+
1 row in set (0.00 sec)SELECT CURDATE(),CURDATE() + 0,CURTIME() + 0,NOW() + 0
FROM DUAL;
#输出
+------------+---------------+---------------+----------------+
| CURDATE()  | CURDATE() + 0 | CURTIME() + 0 | NOW() + 0      |
+------------+---------------+---------------+----------------+
| 2023-05-02 |      20230502 |        211337 | 20230502211337 |
+------------+---------------+---------------+----------------+
1 row in set (0.00 sec)

4.2日期与时间戳的转换

函数	用法
UNIX_TIMESTAMP()	以UNIX时间戳的形式返回当前时间。SELECT UNIX_TIMESTAMP() ->1634348884
UNIX_TIMESTAMP(date)	将时间date以UNIX时间戳的形式返回。
FROM_UNIXTIME(timestamp)	将UNIX时间戳的时间转换为普通格式的时间

举例：

#日期与时间戳的转换
#UNIX_TIMESTAMP(): 以UNIX时间戳(毫秒数)的形式返回当前时间
SELECT UNIX_TIMESTAMP(),UNIX_TIMESTAMP('2023-05-02 21:00:00'),
FROM_UNIXTIME(1635173853),FROM_UNIXTIME(1683032400)
FROM DUAL;
#输出
+------------------+---------------------------------------+---------------------------+---------------------------+
| UNIX_TIMESTAMP() | UNIX_TIMESTAMP('2023-05-02 21:00:00') | FROM_UNIXTIME(1635173853) | FROM_UNIXTIME(1683032400) |
+------------------+---------------------------------------+---------------------------+---------------------------+
|       1683035302 |                            1683032400 | 2021-10-25 22:57:33       | 2023-05-02 21:00:00       |
+------------------+---------------------------------------+---------------------------+---------------------------+
1 row in set (0.00 sec)

4.3 获取月份、星期、星期数、天数等函数

函数	用法
YEAR(date)/MONTH(date) / DAY(date)	返回具体的日期值
HOUR(time)/ MINUTE(time) /SECOND(time)	返回具体的时间值
MONTHNAME(date)	返回月份:January，…
DAYNAME(date)	返回星期几: MONDAY，TUESDAY…SUNDAY
WEEKDAY(date)	返回周几，注意，周1是0，周2是1,。。。周日是6
QUARTER(date)	返回日期对应的季度，范围为1～4
WEEK(date),WEEKOFYEAR(date)	返回一年中的第几周
DAYOFYEAR(date)	返回日期是一年中的第几天
DAYOFMONTH(date)	返回日期位于所在月份的第几天
DAYOFWEEK(date)	返回周几，注意:周日是1，周一是2，。。。周六是7

举例：

#获取月份、星期、星期数、天数等函数
SELECT YEAR(CURDATE()),MONTH(CURDATE()),DAY(CURDATE()),
HOUR(CURTIME()),MINUTE(NOW()),SECOND(SYSDATE())
FROM DUAL;
#输出
+-----------------+------------------+----------------+-----------------+---------------+-------------------+
| YEAR(CURDATE()) | MONTH(CURDATE()) | DAY(CURDATE()) | HOUR(CURTIME()) | MINUTE(NOW()) | SECOND(SYSDATE()) |
+-----------------+------------------+----------------+-----------------+---------------+-------------------+
|            2023 |                5 |              2 |              21 |            15 |                58 |
+-----------------+------------------+----------------+-----------------+---------------+-------------------+
1 row in set (0.00 sec)#MONTHNAME(date) 返回月份：January，..
#DAYNAME(date) 返回星期几：MONDAY，TUESDAY.....SUNDAY
SELECT MONTHNAME('2021-10-26'),DAYNAME('2021-10-26'),WEEKDAY('2021-10-26'),
QUARTER(CURDATE()),WEEK(CURDATE()),DAYOFYEAR(NOW()),
DAYOFMONTH(NOW()),DAYOFWEEK(NOW())
FROM DUAL;
#输出
+-------------------------+-----------------------+-----------------------+--------------------+-----------------+------------------+-------------------+------------------+
| MONTHNAME('2021-10-26') | DAYNAME('2021-10-26') | WEEKDAY('2021-10-26') | QUARTER(CURDATE()) | WEEK(CURDATE()) | DAYOFYEAR(NOW()) | DAYOFMONTH(NOW()) | DAYOFWEEK(NOW()) |
+-------------------------+-----------------------+-----------------------+--------------------+-----------------+------------------+-------------------+------------------+
| October                 | Tuesday               |                     1 |                  2 |              18 |              122 |                 2 |                3 |
+-------------------------+-----------------------+-----------------------+--------------------+-----------------+------------------+-------------------+------------------+
1 row in set (0.01 sec)

4.4 日期的操作函数

函数	用法
EXTRACT(type FROM date)	返回指定日期中特定的部分，type指定返回的值

EXTRACT(type FROM date)函数中type的取值与含义:

举例：

# EXTRACT(type FROM date) 返回指定日期中特定的部分，type指定返回的值
SELECT EXTRACT(SECOND FROM NOW()),EXTRACT(DAY FROM NOW()),
EXTRACT(HOUR_MINUTE FROM NOW()),EXTRACT(QUARTER FROM '2021-05-12')
FROM DUAL;
#输出
+--------------------------+-----------------------+-------------------------------+------------------------------------+
|EXTRACT(SECOND FROM NOW())|EXTRACT(DAY FROM NOW())|EXTRACT(HOUR_MINUTE FROM NOW())| EXTRACT(QUARTER FROM '2021-05-12') |
+--------------------------+-----------------------+-------------------------------+-----------------------------------+
|                       23 |                     2 |                           2121|                             	    2|
+--------------------------+-----------------------+-------------------------------+------------------------------------+
1 row in set (0.00 sec)

4.5 时间和秒钟转换的函数

函数	用法
TIME_TO_SEC(time)	将time转化为秒并返回结果值。转化的公式为:小时3600+分钟60+秒
SEC_TO_TIME(seconds)	将seconds描述转化为包含小时、分钟和秒的时间

举例：

SELECT TIME_TO_SEC(CURTIME()),
SEC_TO_TIME(83355)
FROM DUAL;
#输出
+------------------------+--------------------+
| TIME_TO_SEC(CURTIME()) | SEC_TO_TIME(83355) |
+------------------------+--------------------+
|                  77267 | 23:09:15           |
+------------------------+--------------------+
1 row in set (0.01 sec)