文章目录：
1.字符串函数
以上这些基本上MySQL中常见常用（可能有几个比较偏。。。）的字符串处理函数。
下面我对以上函数做一个简单的案例应用。（注意：MySQL中的字符串索引是从1开始的，这一点和Java是不一样的。）
案例中可能会用到一个employees表，我这里也给出该表的sql语句。（其中外键这部分用到了另外的表，但是在下面的案例中没有涉及，所以问题不大）
/*Table structure for table `employees` */ DROP TABLE IF EXISTS `employees`; CREATE TABLE `employees` ( `employee_id` int(6) NOT NULL DEFAULT '0', `first_name` varchar(20) DEFAULT NULL, `last_name` varchar(25) NOT NULL, `email` varchar(25) NOT NULL, `phone_number` varchar(20) DEFAULT NULL, `hire_date` date NOT NULL, `job_id` varchar(10) NOT NULL, `salary` double(8,2) DEFAULT NULL, `commission_pct` double(2,2) DEFAULT NULL, `manager_id` int(6) DEFAULT NULL, `department_id` int(4) DEFAULT NULL, PRIMARY KEY (`employee_id`), UNIQUE KEY `emp_email_uk` (`email`), UNIQUE KEY `emp_emp_id_pk` (`employee_id`), KEY `emp_dept_fk` (`department_id`), KEY `emp_job_fk` (`job_id`), KEY `emp_manager_fk` (`manager_id`), CONSTRAINT `emp_dept_fk` FOREIGN KEY (`department_id`) REFERENCES `departments` (`department_id`), CONSTRAINT `emp_job_fk` FOREIGN KEY (`job_id`) REFERENCES `jobs` (`job_id`), CONSTRAINT `emp_manager_fk` FOREIGN KEY (`manager_id`) REFERENCES `employees` (`employee_id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; /*Data for the table `employees` */ insert into `employees`(`employee_id`,`first_name`,`last_name`,`email`,`phone_number`,`hire_date`,`job_id`,`salary`,`commission_pct`,`manager_id`,`department_id`) values (100,'Steven','King','SKING','515.123.4567','1987-06-17','AD_PRES',24000.00,NULL,NULL,90),(101,'Neena','Kochhar','NKOCHHAR','515.123.4568','1989-09-21','AD_VP',17000.00,NULL,100,90),(102,'Lex','De Haan','LDEHAAN','515.123.4569','1993-01-13','AD_VP',17000.00,NULL,100,90),(103,'Alexander','Hunold','AHUNOLD','590.423.4567','1990-01-03','IT_PROG',9000.00,NULL,102,60),(104,'Bruce','Ernst','BERNST','590.423.4568','1991-05-21','IT_PROG',6000.00,NULL,103,60),(105,'David','Austin','DAUSTIN','590.423.4569','1997-06-25','IT_PROG',4800.00,NULL,103,60),(106,'Valli','Pataballa','VPATABAL','590.423.4560','1998-02-05','IT_PROG',4800.00,NULL,103,60),(107,'Diana','Lorentz','DLORENTZ','590.423.5567','1999-02-07','IT_PROG',4200.00,NULL,103,60),(108,'Nancy','Greenberg','NGREENBE','515.124.4569','1994-08-17','FI_MGR',12000.00,NULL,101,100),(109,'Daniel','Faviet','DFAVIET','515.124.4169','1994-08-16','FI_ACCOUNT',9000.00,NULL,108,100),(110,'John','Chen','JCHEN','515.124.4269','1997-09-28','FI_ACCOUNT',8200.00,NULL,108,100),(111,'Ismael','Sciarra','ISCIARRA','515.124.4369','1997-09-30','FI_ACCOUNT',7700.00,NULL,108,100),(112,'Jose Manuel','Urman','JMURMAN','515.124.4469','1998-03-07','FI_ACCOUNT',7800.00,NULL,108,100),(113,'Luis','Popp','LPOPP','515.124.4567','1999-12-07','FI_ACCOUNT',6900.00,NULL,108,100),(114,'Den','Raphaely','DRAPHEAL','515.127.4561','1994-12-07','PU_MAN',11000.00,NULL,100,30),(115,'Alexander','Khoo','AKHOO','515.127.4562','1995-05-18','PU_CLERK',3100.00,NULL,114,30),(116,'Shelli','Baida','SBAIDA','515.127.4563','1997-12-24','PU_CLERK',2900.00,NULL,114,30),(117,'Sigal','Tobias','STOBIAS','515.127.4564','1997-07-24','PU_CLERK',2800.00,NULL,114,30),(118,'Guy','Himuro','GHIMURO','515.127.4565','1998-11-15','PU_CLERK',2600.00,NULL,114,30),(119,'Karen','Colmenares','KCOLMENA','515.127.4566','1999-08-10','PU_CLERK',2500.00,NULL,114,30),(120,'Matthew','Weiss','MWEISS','650.123.1234','1996-07-18','ST_MAN',8000.00,NULL,100,50),(121,'Adam','Fripp','AFRIPP','650.123.2234','1997-04-10','ST_MAN',8200.00,NULL,100,50),(122,'Payam','Kaufling','PKAUFLIN','650.123.3234','1995-05-01','ST_MAN',7900.00,NULL,100,50),(123,'Shanta','Vollman','SVOLLMAN','650.123.4234','1997-10-10','ST_MAN',6500.00,NULL,100,50),(124,'Kevin','Mourgos','KMOURGOS','650.123.5234','1999-11-16','ST_MAN',5800.00,NULL,100,50),(125,'Julia','Nayer','JNAYER','650.124.1214','1997-07-16','ST_CLERK',3200.00,NULL,120,50),(126,'Irene','Mikkilineni','IMIKKILI','650.124.1224','1998-09-28','ST_CLERK',2700.00,NULL,120,50),(127,'James','Landry','JLANDRY','650.124.1334','1999-01-14','ST_CLERK',2400.00,NULL,120,50),(128,'Steven','Markle','SMARKLE','650.124.1434','2000-03-08','ST_CLERK',2200.00,NULL,120,50),(129,'Laura','Bissot','LBISSOT','650.124.5234','1997-08-20','ST_CLERK',3300.00,NULL,121,50),(130,'Mozhe','Atkinson','MATKINSO','650.124.6234','1997-10-30','ST_CLERK',2800.00,NULL,121,50),(131,'James','Marlow','JAMRLOW','650.124.7234','1997-02-16','ST_CLERK',2500.00,NULL,121,50),(132,'TJ','Olson','TJOLSON','650.124.8234','1999-04-10','ST_CLERK',2100.00,NULL,121,50),(133,'Jason','Mallin','JMALLIN','650.127.1934','1996-06-14','ST_CLERK',3300.00,NULL,122,50),(134,'Michael','Rogers','MROGERS','650.127.1834','1998-08-26','ST_CLERK',2900.00,NULL,122,50),(135,'Ki','Gee','KGEE','650.127.1734','1999-12-12','ST_CLERK',2400.00,NULL,122,50),(136,'Hazel','Philtanker','HPHILTAN','650.127.1634','2000-02-06','ST_CLERK',2200.00,NULL,122,50),(137,'Renske','Ladwig','RLADWIG','650.121.1234','1995-07-14','ST_CLERK',3600.00,NULL,123,50),(138,'Stephen','Stiles','SSTILES','650.121.2034','1997-10-26','ST_CLERK',3200.00,NULL,123,50),(139,'John','Seo','JSEO','650.121.2019','1998-02-12','ST_CLERK',2700.00,NULL,123,50),(140,'Joshua','Patel','JPATEL','650.121.1834','1998-04-06','ST_CLERK',2500.00,NULL,123,50),(141,'Trenna','Rajs','TRAJS','650.121.8009','1995-10-17','ST_CLERK',3500.00,NULL,124,50),(142,'Curtis','Davies','CDAVIES','650.121.2994','1997-01-29','ST_CLERK',3100.00,NULL,124,50),(143,'Randall','Matos','RMATOS','650.121.2874','1998-03-15','ST_CLERK',2600.00,NULL,124,50),(144,'Peter','Vargas','PVARGAS','650.121.2004','1998-07-09','ST_CLERK',2500.00,NULL,124,50),(145,'John','Russell','JRUSSEL','011.44.1344.429268','1996-10-01','SA_MAN',14000.00,0.40,100,80),(146,'Karen','Partners','KPARTNER','011.44.1344.467268','1997-01-05','SA_MAN',13500.00,0.30,100,80),(147,'Alberto','Errazuriz','AERRAZUR','011.44.1344.429278','1997-03-10','SA_MAN',12000.00,0.30,100,80),(148,'Gerald','Cambrault','GCAMBRAU','011.44.1344.619268','1999-10-15','SA_MAN',11000.00,0.30,100,80),(149,'Eleni','Zlotkey','EZLOTKEY','011.44.1344.429018','2000-01-29','SA_MAN',10500.00,0.20,100,80),(150,'Peter','Tucker','PTUCKER','011.44.1344.129268','1997-01-30','SA_REP',10000.00,0.30,145,80),(151,'David','Bernstein','DBERNSTE','011.44.1344.345268','1997-03-24','SA_REP',9500.00,0.25,145,80),(152,'Peter','Hall','PHALL','011.44.1344.478968','1997-08-20','SA_REP',9000.00,0.25,145,80),(153,'Christopher','Olsen','COLSEN','011.44.1344.498718','1998-03-30','SA_REP',8000.00,0.20,145,80),(154,'Nanette','Cambrault','NCAMBRAU','011.44.1344.987668','1998-12-09','SA_REP',7500.00,0.20,145,80),(155,'Oliver','Tuvault','OTUVAULT','011.44.1344.486508','1999-11-23','SA_REP',7000.00,0.15,145,80),(156,'Janette','King','JKING','011.44.1345.429268','1996-01-30','SA_REP',10000.00,0.35,146,80),(157,'Patrick','Sully','PSULLY','011.44.1345.929268','1996-03-04','SA_REP',9500.00,0.35,146,80),(158,'Allan','McEwen','AMCEWEN','011.44.1345.829268','1996-08-01','SA_REP',9000.00,0.35,146,80),(159,'Lindsey','Smith','LSMITH','011.44.1345.729268','1997-03-10','SA_REP',8000.00,0.30,146,80),(160,'Louise','Doran','LDORAN','011.44.1345.629268','1997-12-15','SA_REP',7500.00,0.30,146,80),(161,'Sarath','Sewall','SSEWALL','011.44.1345.529268','1998-11-03','SA_REP',7000.00,0.25,146,80),(162,'Clara','Vishney','CVISHNEY','011.44.1346.129268','1997-11-11','SA_REP',10500.00,0.25,147,80),(163,'Danielle','Greene','DGREENE','011.44.1346.229268','1999-03-19','SA_REP',9500.00,0.15,147,80),(164,'Mattea','Marvins','MMARVINS','011.44.1346.329268','2000-01-24','SA_REP',7200.00,0.10,147,80),(165,'David','Lee','DLEE','011.44.1346.529268','2000-02-23','SA_REP',6800.00,0.10,147,80),(166,'Sundar','Ande','SANDE','011.44.1346.629268','2000-03-24','SA_REP',6400.00,0.10,147,80),(167,'Amit','Banda','ABANDA','011.44.1346.729268','2000-04-21','SA_REP',6200.00,0.10,147,80),(168,'Lisa','Ozer','LOZER','011.44.1343.929268','1997-03-11','SA_REP',11500.00,0.25,148,80),(169,'Harrison','Bloom','HBLOOM','011.44.1343.829268','1998-03-23','SA_REP',10000.00,0.20,148,80),(170,'Tayler','Fox','TFOX','011.44.1343.729268','1998-01-24','SA_REP',9600.00,0.20,148,80),(171,'William','Smith','WSMITH','011.44.1343.629268','1999-02-23','SA_REP',7400.00,0.15,148,80),(172,'Elizabeth','Bates','EBATES','011.44.1343.529268','1999-03-24','SA_REP',7300.00,0.15,148,80),(173,'Sundita','Kumar','SKUMAR','011.44.1343.329268','2000-04-21','SA_REP',6100.00,0.10,148,80),(174,'Ellen','Abel','EABEL','011.44.1644.429267','1996-05-11','SA_REP',11000.00,0.30,149,80),(175,'Alyssa','Hutton','AHUTTON','011.44.1644.429266','1997-03-19','SA_REP',8800.00,0.25,149,80),(176,'Jonathon','Taylor','JTAYLOR','011.44.1644.429265','1998-03-24','SA_REP',8600.00,0.20,149,80),(177,'Jack','Livingston','JLIVINGS','011.44.1644.429264','1998-04-23','SA_REP',8400.00,0.20,149,80),(178,'Kimberely','Grant','KGRANT','011.44.1644.429263','1999-05-24','SA_REP',7000.00,0.15,149,NULL),(179,'Charles','Johnson','CJOHNSON','011.44.1644.429262','2000-01-04','SA_REP',6200.00,0.10,149,80),(180,'Winston','Taylor','WTAYLOR','650.507.9876','1998-01-24','SH_CLERK',3200.00,NULL,120,50),(181,'Jean','Fleaur','JFLEAUR','650.507.9877','1998-02-23','SH_CLERK',3100.00,NULL,120,50),(182,'Martha','Sullivan','MSULLIVA','650.507.9878','1999-06-21','SH_CLERK',2500.00,NULL,120,50),(183,'Girard','Geoni','GGEONI','650.507.9879','2000-02-03','SH_CLERK',2800.00,NULL,120,50),(184,'Nandita','Sarchand','NSARCHAN','650.509.1876','1996-01-27','SH_CLERK',4200.00,NULL,121,50),(185,'Alexis','Bull','ABULL','650.509.2876','1997-02-20','SH_CLERK',4100.00,NULL,121,50),(186,'Julia','Dellinger','JDELLING','650.509.3876','1998-06-24','SH_CLERK',3400.00,NULL,121,50),(187,'Anthony','Cabrio','ACABRIO','650.509.4876','1999-02-07','SH_CLERK',3000.00,NULL,121,50),(188,'Kelly','Chung','KCHUNG','650.505.1876','1997-06-14','SH_CLERK',3800.00,NULL,122,50),(189,'Jennifer','Dilly','JDILLY','650.505.2876','1997-08-13','SH_CLERK',3600.00,NULL,122,50),(190,'Timothy','Gates','TGATES','650.505.3876','1998-07-11','SH_CLERK',2900.00,NULL,122,50),(191,'Randall','Perkins','RPERKINS','650.505.4876','1999-12-19','SH_CLERK',2500.00,NULL,122,50),(192,'Sarah','Bell','SBELL','650.501.1876','1996-02-04','SH_CLERK',4000.00,NULL,123,50),(193,'Britney','Everett','BEVERETT','650.501.2876','1997-03-03','SH_CLERK',3900.00,NULL,123,50),(194,'Samuel','McCain','SMCCAIN','650.501.3876','1998-07-01','SH_CLERK',3200.00,NULL,123,50),(195,'Vance','Jones','VJONES','650.501.4876','1999-03-17','SH_CLERK',2800.00,NULL,123,50),(196,'Alana','Walsh','AWALSH','650.507.9811','1998-04-24','SH_CLERK',3100.00,NULL,124,50),(197,'Kevin','Feeney','KFEENEY','650.507.9822','1998-05-23','SH_CLERK',3000.00,NULL,124,50),(198,'Donald','OConnell','DOCONNEL','650.507.9833','1999-06-21','SH_CLERK',2600.00,NULL,124,50),(199,'Douglas','Grant','DGRANT','650.507.9844','2000-01-13','SH_CLERK',2600.00,NULL,124,50),(200,'Jennifer','Whalen','JWHALEN','515.123.4444','1987-09-17','AD_ASST',4400.00,NULL,101,10),(201,'Michael','Hartstein','MHARTSTE','515.123.5555','1996-02-17','MK_MAN',13000.00,NULL,100,20),(202,'Pat','Fay','PFAY','603.123.6666','1997-08-17','MK_REP',6000.00,NULL,201,20),(203,'Susan','Mavris','SMAVRIS','515.123.7777','1994-06-07','HR_REP',6500.00,NULL,101,40),(204,'Hermann','Baer','HBAER','515.123.8888','1994-06-07','PR_REP',10000.00,NULL,101,70),(205,'Shelley','Higgins','SHIGGINS','515.123.8080','1994-06-07','AC_MGR',12000.00,NULL,101,110),(206,'William','Gietz','WGIETZ','515.123.8181','1994-06-07','AC_ACCOUNT',8300.00,NULL,205,110);
字符串函数的相关应用：👇👇👇
SELECT ASCII('Abcdfsf'), CHAR_LENGTH('hello'), CHAR_LENGTH('小哥'), LENGTH('hello'), LENGTH('小哥') FROM DUAL;
SELECT CONCAT(emp.last_name,' worked for ',mgr.last_name) "details" FROM employees emp JOIN employees mgr ON emp.manager_id = mgr.employee_id;
SELECT CONCAT_WS('-','hello','world','hello','henan') FROM DUAL;
SELECT INSERT('ArrayList',1,5,'Linked'), REPLACE('hello','l','x') FROM DUAL;
SELECT UPPER('HeLlO'), LOWER('HeLlO') FROM DUAL;
SELECT LEFT('HashMap',4), RIGHT('StringBuilder',7), RIGHT('SpringBoot',20) FROM DUAL;
SELECT employee_id, last_name, LPAD(salary,10,' '), RPAD(salary,10,' ') FROM employees;
SELECT CONCAT('---', LTRIM(' h e l l o '), '***'), CONCAT('---', RTRIM(' h e l l o '), '***'), CONCAT('---', TRIM(' h e l l o '), '***') FROM DUAL;
SELECT REPEAT('Java',3), LENGTH(SPACE(5)), STRCMP('abc', 'abe'), STRCMP('abc', 'abc'), STRCMP('abc', 'aba') FROM DUAL;
SELECT SUBSTR('LinkedHashSet', 7, 7), LOCATE('Stream','InputStreamStream') 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;
2.日期时间函数
2.1 获取日期时间
SELECT CURDATE(), CURRENT_DATE() FROM DUAL;
SELECT CURTIME(), CURRENT_TIME() FROM DUAL;
SELECT NOW(), SYSDATE(), LOCALTIME(), LOCALTIMESTAMP(), CURRENT_TIMESTAMP() FROM DUAL;
SELECT UTC_DATE(), UTC_TIME() FROM DUAL;
SELECT CURDATE(),CURDATE() + 0,CURTIME() + 0,NOW() + 0 FROM DUAL;
2.2 日期与时间戳的转换
SELECT UNIX_TIMESTAMP(), UNIX_TIMESTAMP('2020-11-22 05:33:58'), FROM_UNIXTIME(1648782044), FROM_UNIXTIME(1605994438) FROM DUAL;
2.3 获取年月日、时分秒、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('2021-10-24'), DAYNAME('2021-10-24'), WEEKDAY('2021-10-24'), QUARTER(CURDATE()), WEEK(NOW()), DAYOFYEAR(SYSDATE()), DAYOFMONTH(LOCALTIME()), DAYOFWEEK(LOCALTIMESTAMP()) FROM DUAL;
SELECT EXTRACT(YEAR FROM NOW()), EXTRACT(MONTH FROM NOW()), EXTRACT(DAY FROM NOW()), EXTRACT(DAY_HOUR FROM SYSDATE()), EXTRACT(HOUR_MINUTE FROM SYSDATE()) FROM DUAL;
2.4 时间和秒钟的转换
SELECT TIME_TO_SEC(CURTIME()), SEC_TO_TIME(40999) FROM DUAL;
2.5 计算日期和时间的函数
SELECT NOW(), DATE_ADD(NOW(),INTERVAL 1 YEAR), DATE_SUB(NOW(),INTERVAL 2 MONTH), ADDDATE('2021-10-21 20:32:12',INTERVAL '2_3' DAY_HOUR), SUBDATE('2021-10-21 23:32:12',INTERVAL '3_20' HOUR_MINUTE) 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 DUAL;
SELECT 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;
2.6 日期的格式化与解析
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;
SELECT STR_TO_DATE('2021-October-25th 13:37:30','%Y-%M-%D %H:%i:%S'), STR_TO_DATE('2015-04-22 15:47:06','%Y-%m-%d %H:%i:%s') FROM DUAL;
3.流程控制函数
流程处理函数可以根据不同的条件，执行不同的处理流程，可以在SQL语句中实现不同的条件选择。MySQL中的流程处理函数主要包括IF()、IFNULL()和CASE()函数。
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 + IFNULL(commission_pct,0)) "annual_sal" FROM employees;
SELECT last_name, salary, CASE WHEN salary >= 15000 THEN '高级工程师' WHEN salary >= 10000 THEN '中级工程师' WHEN salary >= 8000 THEN '初级工程师' ELSE '小白' END "details" FROM employees;
SELECT employee_id, last_name, department_id, salary, CASE department_id WHEN 10 THEN salary * 1.1 WHEN '20' THEN salary * 1.3 WHEN 30 THEN salary * 1.5 ELSE salary * 2 END "details" FROM employees;
