1.数值函数

1.1 基本的操作

SELECT ABS(-123),ABS(32),SIGN(-23),SIGN(43),PI(),CEIL(32.32),CEILING(-43.23),FLOOR(32.32),
FLOOR(-43.23),MOD(12,5),12 MOD 5,12 % 5
FROM DUAL;

1.2 取随机数

SELECT RAND(),RAND(),RAND(10),RAND(10),RAND(-1),RAND(-1)
FROM DUAL;

1.3 四舍五入，截断操作

SELECT ROUND(123.556),ROUND(123.456,0),ROUND(123.456,1),ROUND(123.456,2),
ROUND(123.456,-1),ROUND(153.456,-2)
FROM DUAL;

SELECT TRUNCATE(123.456,0),TRUNCATE(123.496,1),TRUNCATE(129.45,-1)
FROM DUAL;

1.4 单行函数可以嵌套

SELECT TRUNCATE(ROUND(123.456,2),0)
FROM DUAL;

1.5 角度与弧度的互换

SELECT RADIANS(30),RADIANS(45),RADIANS(60),RADIANS(90),
DEGREES(2*PI()),DEGREES(RADIANS(60))
FROM DUAL;

1.6 三角函数

SELECT SIN(RADIANS(30)),DEGREES(ASIN(1)),TAN(RADIANS(45)),DEGREES(ATAN(1))
FROM DUAL;

1.7 指数和对数

SELECT POW(2,5),POWER(2,4),EXP(2)
FROM DUAL;

SELECT LN(EXP(2)),LOG(EXP(2)),LOG10(10),LOG2(4)
FROM DUAL;

1.8 进制间的转换

SELECT BIN(10),HEX(10),OCT(10),CONV(10,10,8)
FROM DUAL;

2. 字符串函数

SELECT ASCII('Abcdfsf'),CHAR_LENGTH('hello'),CHAR_LENGTH('我们'),
LENGTH('hello'),LENGTH('我们')
FROM DUAL;

2.1 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;

SELECT CONCAT_WS('-','hello','world','hello','beijing')
FROM DUAL;

#字符串的索引是从1开始的！
SELECT INSERT('helloworld',2,3,'aaaaa'),REPLACE('hello','lol','mmm')
FROM DUAL;

SELECT UPPER('HelLo'),LOWER('HelLo')
FROM DUAL;

SELECT last_name,salary
FROM employees
WHERE LOWER(last_name) = 'King';

SELECT LEFT('hello',2),RIGHT('hello',3),RIGHT('hello',13)
FROM DUAL;

2.2 LPAD:实现右对齐效果

2.3 RPAD:实现左对齐效果

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

SELECT CONCAT('---',LTRIM('    h  el  lo   '),'***'),
TRIM('oo' FROM 'ooheollo')
FROM DUAL;

SELECT REPEAT('hello',4),LENGTH(SPACE(5)),STRCMP('abc','abe')
FROM DUAL;

SELECT SUBSTR('hello',2,2),LOCATE('lll','hello')
FROM DUAL;

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;

SELECT employee_id,NULLIF(LENGTH(first_name),LENGTH(last_name)) "compare"
FROM employees;

3. 日期和时间函数

3.1 获取日期、时间

SELECT CURDATE(),CURRENT_DATE(),CURTIME(),NOW(),SYSDATE(),
UTC_DATE(),UTC_TIME()
FROM DUAL;

SELECT CURDATE(),CURDATE() + 0,CURTIME() + 0,NOW() + 0
FROM DUAL;

3.2 日期与时间戳的转换

SELECT UNIX_TIMESTAMP(),UNIX_TIMESTAMP('2021-10-01 12:12:32'),
FROM_UNIXTIME(1635173853),FROM_UNIXTIME(1633061552)
FROM DUAL;

3.3 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;

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;

3.4 日期的操作函数

SELECT EXTRACT(SECOND FROM NOW()),EXTRACT(DAY FROM NOW()),
EXTRACT(HOUR_MINUTE FROM NOW()),EXTRACT(QUARTER FROM '2021-05-12')
FROM DUAL;

3.5 时间和秒钟转换的函数

SELECT TIME_TO_SEC(CURTIME()),
SEC_TO_TIME(83355)
FROM DUAL;

3.6 计算日期和时间的函数

SELECT NOW(),DATE_ADD(NOW(),INTERVAL 1 YEAR),
DATE_ADD(NOW(),INTERVAL -1 YEAR),
DATE_SUB(NOW(),INTERVAL 1 YEAR)
FROM DUAL;

SELECT DATE_ADD(NOW(), INTERVAL 1 DAY) AS col1,DATE_ADD('2021-10-21 23:32:12',INTERVAL 1 SECOND) AS col2,
ADDDATE('2021-10-21 23:32:12',INTERVAL 1 SECOND) AS col3,
DATE_ADD('2021-10-21 23:32:12',INTERVAL '1_1' MINUTE_SECOND) AS col4,
DATE_ADD(NOW(), INTERVAL -1 YEAR) AS col5, #可以是负数
DATE_ADD(NOW(), INTERVAL '1_1' YEAR_MONTH) AS col6 #需要单引号
FROM DUAL;

SELECT ADDTIME(NOW(),20),SUBTIME(NOW(),30),SUBTIME(NOW(),'1:1:3'),DATEDIFF(NOW(),'2021-10-01'),
TIMEDIFF(NOW(),'2021-10-25 22:10:10'),FROM_DAYS(366),TO_DAYS('0000-12-25'),
LAST_DAY(NOW()),MAKEDATE(YEAR(NOW()),32),MAKETIME(10,21,23),PERIOD_ADD(20200101010101,10)
FROM DUAL;

3.7 日期的格式化与解析

格式化：日期 —> 字符串
解析： 字符串 ----> 日期
此时我们谈的是日期的显式格式化和解析
之前，我们接触过隐式的格式化或解析

SELECT *
FROM employees
WHERE hire_date = '1993-01-13';

#格式化：

SELECT DATE_FORMAT(CURDATE(),'%Y-%M-%D'),
DATE_FORMAT(NOW(),'%Y-%m-%d'),TIME_FORMAT(CURTIME(),'%h:%i:%S'),
DATE_FORMAT(NOW(),'%Y-%M-%D %h:%i:%S %W %w %T %r')
FROM DUAL;

解析：格式化的逆过程

SELECT STR_TO_DATE('2021-October-25th 11:37:30 Monday 1','%Y-%M-%D %h:%i:%S %W %w')
FROM DUAL;

SELECT GET_FORMAT(DATE,'USA')
FROM DUAL;

SELECT DATE_FORMAT(CURDATE(),GET_FORMAT(DATE,'USA'))
FROM DUAL;

4.流程控制函数

4.1 IF(VALUE,VALUE1,VALUE2)

SELECT last_name,salary,IF(salary >= 6000,'高工资','低工资') "details"
FROM employees;

SELECT last_name,commission_pct,IF(commission_pct IS NOT NULL,commission_pct,0) "details",
salary * 12 * (1 + IF(commission_pct IS NOT NULL,commission_pct,0)) "annual_sal"
FROM employees;

4.2 IFNULL(VALUE1,VALUE2):看做是IF(VALUE,VALUE1,VALUE2)的特殊情况

SELECT last_name,commission_pct,IFNULL(commission_pct,0) "details"
FROM employees;

4.3 CASE WHEN … THEN …WHEN … THEN … ELSE … END

类似于java的if … else if … else if … else

SELECT last_name,salary,CASE WHEN salary >= 15000 THEN '白骨精' 
			     WHEN salary >= 10000 THEN '潜力股'
			     WHEN salary >= 8000 THEN '小屌丝'
			     ELSE '草根' END "details",department_id
FROM employees;

SELECT last_name,salary,CASE WHEN salary >= 15000 THEN '白骨精' 
			     WHEN salary >= 10000 THEN '潜力股'
			     WHEN salary >= 8000 THEN '小屌丝'
			     END "details"
FROM employees;

4.4 CASE … WHEN … THEN … WHEN … THEN … ELSE … END

类似于java的swich … case…
练习1
查询部门号为 10,20, 30 的员工信息,
若部门号为 10, 则打印其工资的 1.1 倍,
20 号部门, 则打印其工资的 1.2 倍,
30 号部门,打印其工资的 1.3 倍数,
其他部门,打印其工资的 1.4 倍数

SELECT employee_id,last_name,department_id,salary,CASE department_id WHEN 10 THEN salary * 1.1
								     WHEN 20 THEN salary * 1.2
								     WHEN 30 THEN salary * 1.3
								     ELSE salary * 1.4 END "details"
FROM employees;

练习2
查询部门号为 10,20, 30 的员工信息,
若部门号为 10, 则打印其工资的 1.1 倍,
20 号部门, 则打印其工资的 1.2 倍,
30 号部门打印其工资的 1.3 倍数

SELECT employee_id,last_name,department_id,salary,CASE department_id WHEN 10 THEN salary * 1.1
								     WHEN 20 THEN salary * 1.2
								     WHEN 30 THEN salary * 1.3
								     END "details"
FROM employees
WHERE department_id IN (10,20,30);

5. 加密与解密的函数

PASSWORD()在mysql8.0中弃用。

SELECT MD5('mysql'),SHA('mysql'),MD5(MD5('mysql'))
FROM DUAL;

ENCODE()\DECODE() 在mysql8.0中弃用。

SELECT ENCODE('atguigu','mysql'),DECODE(ENCODE('atguigu','mysql'),'mysql')
FROM DUAL;

6. MySQL信息函数

SELECT VERSION(),CONNECTION_ID(),DATABASE(),SCHEMA(),
USER(),CURRENT_USER(),CHARSET('尚硅谷'),COLLATION('尚硅谷')
FROM DUAL;

7. 其他函数

#如果n的值小于或者等于0，则只保留整数部分

SELECT FORMAT(123.125,2),FORMAT(123.125,0),FORMAT(123.125,-2)
FROM DUAL;

SELECT CONV(16, 10, 2), CONV(8888,10,16), CONV(NULL, 10, 2)
FROM DUAL;

以“192.168.1.100”为例，计算方式为192乘以256的3次方，加上168乘以256的2次方，加上1乘以256，再加上100。

SELECT INET_ATON('192.168.1.100'),INET_NTOA(3232235876)
FROM DUAL;

BENCHMARK()用于测试表达式的执行效率

SELECT BENCHMARK(100000,MD5('mysql'))
FROM DUAL;

CONVERT():可以实现字符集的转换

SELECT CHARSET('atguigu'),CHARSET(CONVERT('atguigu' USING 'gbk'))
FROM DUAL;

8.单行函数-练习

1.显示系统时间(注：日期+时间)

SELECT NOW() 
FROM DUAL;

2.查询员工号，姓名，工资，以及工资提高百分之20%后的结果（new salary）

SELECT employee_id, last_name, salary, salary * 1.2 "new salary" 
FROM employees;

3.将员工的姓名按首字母排序，并写出姓名的长度（length）

SELECT last_name, LENGTH(last_name) 
FROM employees 
ORDER BY last_name 
DESC;

4.查询员工id,last_name,salary，并作为一个列输出，别名为OUT_PUT

SELECT CONCAT(employee_id, ',' , last_name , ',', salary) OUT_PUT 
FROM employees;

5.查询公司各员工工作的年数、工作的天数，并按工作年数的降序排序。 commission_pct不为空

SELECT DATEDIFF(NOW(),e.hire_date)/365 work_years,DATEDIFF(NOW(),e.hire_date) work_days
from employees e
ORDER BY work_years DESC;

6.查询员工姓名，hire_date , department_id，
满足以下条件：雇用时间在 1997年之后，department_id 为80 或 90 或110, commission_pct不为空

SELECT e.last_name,e.hire_date,e.department_id
from employees e
-- where hire_date>='1997-01-1' 
-- WHERE DATE_FORMAT(NOW(),'%Y')>= '1997'
where hire_date>=STR_TO_DATE('1997-01-01','%Y-%m-%d')
and department_id in(80,90,110) 
AND commission_pct is not null;

7.查询公司中入职超过10000天的员工姓名、入职时间

SELECT last_name,hire_date
from employees
-- where DATEDIFF(NOW(),hire_date)>10000;
WHERE TO_DAYS(NOW())-TO_DAYS(hire_date)>10000;

8.做一个查询，产生下面的结果

SELECT CONCAT(last_name,' earns ',salary,' monthly but wants ',TRUNCATE(salary*3,0)) "Dream Salary"
from employees;

SELECT last_name Last_name, job_id Job_id,CASE job_id when 'AD_PRES' then 'A'
                                                      when 'ST_MAN' then 'A'
                                                      when 'IT_PROG' then 'A'
                                                      when ' SA_REP' then 'A'
                                                      ELSE 'E'
                                                      END "Grade"
FROM employees;