mysql子查询
子查询
所谓子查询，就是嵌套在其他查询中的查询
某些情况下，当进行一个查询时，需要的条件或数据要用另外一个 select 语句的结果，这个时候，就要用到子查询。
定义：执行顺序方面先于当前查询执行的，并且是嵌套在当前查询中的查询叫做子查询
MySQL 在处理上例的 SELECT 语句时，执行流程为：先执行子查询，再执行父查询。
子查询应当始终放在括号内
子查询的支持是从 4.1 版本引入的。MySQL 的早期版本不支持子查询。
子查询分类
按照返回结果的行列数分类
- 标量子查询（结果集只有一行一列）
- 列子查询（结果集只有一列多行）
- 行子查询（结果集有一行多列）
- 表子查询（结果集一般为多列多行）
按子查询出现在主查询的位置分类
- select后面：仅支持标量子查询
- from后面：支持表子查询
- where或者having后面：支持标量子查询（单行单列）、列子查询（单列多行）、行子查询（多列多行）
- exists后面（即相关子查询）： 表子查询（多行多列）
测试数据
测试文件路径：https://files.cnblogs.com/files/shipment/employees.zip
或者复制下面的，保存在文本中source过来
drop database if exists `employees`;
create database `employees`;
USE `employees`;
/*部门表*/
DROP TABLE IF EXISTS `departments`;
CREATE TABLE `departments` (
`department_id` int(4) NOT NULL AUTO_INCREMENT comment '部门id',
`department_name` varchar(3) DEFAULT NULL comment '部门名称',
`manager_id` int(6) DEFAULT NULL comment '管理者id',
`location_id` int(4) DEFAULT NULL comment '部门位置id，来源于表locations中的location_id',
PRIMARY KEY (`department_id`),
KEY `loc_id_fk` (`location_id`)
) ENGINE=InnoDB AUTO_INCREMENT=271 comment '部门表';
insert into `departments`(`department_id`,`department_name`,`manager_id`,`location_id`) values (10,'Adm',200,1700),(20,'Mar',201,1800),(30,'Pur',114,1700),(40,'Hum',203,2400),(50,'Shi',121,1500),(60,'IT',103,1400),(70,'Pub',204,2700),(80,'Sal',145,2500),(90,'Exe',100,1700),(100,'Fin',108,1700),(110,'Acc',205,1700),(120,'Tre',NULL,1700),(130,'Cor',NULL,1700),(140,'Con',NULL,1700),(150,'Sha',NULL,1700),(160,'Ben',NULL,1700),(170,'Man',NULL,1700),(180,'Con',NULL,1700),(190,'Con',NULL,1700),(200,'Ope',NULL,1700),(210,'IT ',NULL,1700),(220,'NOC',NULL,1700),(230,'IT ',NULL,1700),(240,'Gov',NULL,1700),(250,'Ret',NULL,1700),(260,'Rec',NULL,1700),(270,'Pay',NULL,1700);
/*员工表*/
DROP TABLE IF EXISTS `employees`;
CREATE TABLE `employees` (
`employee_id` int(6) NOT NULL AUTO_INCREMENT comment '员工id',
`first_name` varchar(20) DEFAULT NULL comment '名',
`last_name` varchar(25) DEFAULT NULL comment '姓',
`email` varchar(25) DEFAULT NULL comment '电子邮箱',
`phone_number` varchar(20) DEFAULT NULL comment '手机',
`job_id` varchar(10) DEFAULT NULL comment '职位id，来源于jobs表中的job_id',
`salary` double(10,2) DEFAULT NULL comment '薪水',
`commission_pct` double(4,2) DEFAULT NULL comment '佣金百分比',
`manager_id` int(6) DEFAULT NULL comment '上级id',
`department_id` int(4) DEFAULT NULL comment '所属部门id，来源于departments中的department_id',
`hiredate` datetime DEFAULT NULL comment '入职日期',
PRIMARY KEY (`employee_id`)
) ENGINE=InnoDB AUTO_INCREMENT=207 comment '员工表';
insert into `employees`(`employee_id`,`first_name`,`last_name`,`email`,`phone_number`,`job_id`,`salary`,`commission_pct`,`manager_id`,`department_id`,`hiredate`) values (100,'Steven','K_ing','SKING','515.123.4567','AD_PRES',24000.00,NULL,NULL,90,'1992-04-03 00:00:00'),(101,'Neena','Kochhar','NKOCHHAR','515.123.4568','AD_VP',17000.00,NULL,100,90,'1992-04-03 00:00:00'),(102,'Lex','De Haan','LDEHAAN','515.123.4569','AD_VP',17000.00,NULL,100,90,'1992-04-03 00:00:00'),(103,'Alexander','Hunold','AHUNOLD','590.423.4567','IT_PROG',9000.00,NULL,102,60,'1992-04-03 00:00:00'),(104,'Bruce','Ernst','BERNST','590.423.4568','IT_PROG',6000.00,NULL,103,60,'1992-04-03 00:00:00'),(105,'David','Austin','DAUSTIN','590.423.4569','IT_PROG',4800.00,NULL,103,60,'1998-03-03 00:00:00'),(106,'Valli','Pataballa','VPATABAL','590.423.4560','IT_PROG',4800.00,NULL,103,60,'1998-03-03 00:00:00'),(107,'Diana','Lorentz','DLORENTZ','590.423.5567','IT_PROG',4200.00,NULL,103,60,'1998-03-03 00:00:00'),(108,'Nancy','Greenberg','NGREENBE','515.124.4569','FI_MGR',12000.00,NULL,101,100,'1998-03-03 00:00:00'),(109,'Daniel','Faviet','DFAVIET','515.124.4169','FI_ACCOUNT',9000.00,NULL,108,100,'1998-03-03 00:00:00'),(110,'John','Chen','JCHEN','515.124.4269','FI_ACCOUNT',8200.00,NULL,108,100,'2000-09-09 00:00:00'),(111,'Ismael','Sciarra','ISCIARRA','515.124.4369','FI_ACCOUNT',7700.00,NULL,108,100,'2000-09-09 00:00:00'),(112,'Jose Manuel','Urman','JMURMAN','515.124.4469','FI_ACCOUNT',7800.00,NULL,108,100,'2000-09-09 00:00:00'),(113,'Luis','Popp','LPOPP','515.124.4567','FI_ACCOUNT',6900.00,NULL,108,100,'2000-09-09 00:00:00'),(114,'Den','Raphaely','DRAPHEAL','515.127.4561','PU_MAN',11000.00,NULL,100,30,'2000-09-09 00:00:00'),(115,'Alexander','Khoo','AKHOO','515.127.4562','PU_CLERK',3100.00,NULL,114,30,'2000-09-09 00:00:00'),(116,'Shelli','Baida','SBAIDA','515.127.4563','PU_CLERK',2900.00,NULL,114,30,'2000-09-09 00:00:00'),(117,'Sigal','Tobias','STOBIAS','515.127.4564','PU_CLERK',2800.00,NULL,114,30,'2000-09-09 00:00:00'),(118,'Guy','Himuro','GHIMURO','515.127.4565','PU_CLERK',2600.00,NULL,114,30,'2000-09-09 00:00:00'),(119,'Karen','Colmenares','KCOLMENA','515.127.4566','PU_CLERK',2500.00,NULL,114,30,'2000-09-09 00:00:00'),(120,'Matthew','Weiss','MWEISS','650.123.1234','ST_MAN',8000.00,NULL,100,50,'2004-02-06 00:00:00'),(121,'Adam','Fripp','AFRIPP','650.123.2234','ST_MAN',8200.00,NULL,100,50,'2004-02-06 00:00:00'),(122,'Payam','Kaufling','PKAUFLIN','650.123.3234','ST_MAN',7900.00,NULL,100,50,'2004-02-06 00:00:00'),(123,'Shanta','Vollman','SVOLLMAN','650.123.4234','ST_MAN',6500.00,NULL,100,50,'2004-02-06 00:00:00'),(124,'Kevin','Mourgos','KMOURGOS','650.123.5234','ST_MAN',5800.00,NULL,100,50,'2004-02-06 00:00:00'),(125,'Julia','Nayer','JNAYER','650.124.1214','ST_CLERK',3200.00,NULL,120,50,'2004-02-06 00:00:00'),(126,'Irene','Mikkilineni','IMIKKILI','650.124.1224','ST_CLERK',2700.00,NULL,120,50,'2004-02-06 00:00:00'),(127,'James','Landry','JLANDRY','650.124.1334','ST_CLERK',2400.00,NULL,120,50,'2004-02-06 00:00:00'),(128,'Steven','Markle','SMARKLE','650.124.1434','ST_CLERK',2200.00,NULL,120,50,'2004-02-06 00:00:00'),(129,'Laura','Bissot','LBISSOT','650.124.5234','ST_CLERK',3300.00,NULL,121,50,'2004-02-06 00:00:00'),(130,'Mozhe','Atkinson','MATKINSO','650.124.6234','ST_CLERK',2800.00,NULL,121,50,'2004-02-06 00:00:00'),(131,'James','Marlow','JAMRLOW','650.124.7234','ST_CLERK',2500.00,NULL,121,50,'2004-02-06 00:00:00'),(132,'TJ','Olson','TJOLSON','650.124.8234','ST_CLERK',2100.00,NULL,121,50,'2004-02-06 00:00:00'),(133,'Jason','Mallin','JMALLIN','650.127.1934','ST_CLERK',3300.00,NULL,122,50,'2004-02-06 00:00:00'),(134,'Michael','Rogers','MROGERS','650.127.1834','ST_CLERK',2900.00,NULL,122,50,'2002-12-23 00:00:00'),(135,'Ki','Gee','KGEE','650.127.1734','ST_CLERK',2400.00,NULL,122,50,'2002-12-23 00:00:00'),(136,'Hazel','Philtanker','HPHILTAN','650.127.1634','ST_CLERK',2200.00,NULL,122,50,'2002-12-23 00:00:00'),(137,'Renske','Ladwig','RLADWIG','650.121.1234','ST_CLERK',3600.00,NULL,123,50,'2002-12-23 00:00:00'),(138,'Stephen','Stiles','SSTILES','650.121.2034','ST_CLERK',3200.00,NULL,123,50,'2002-12-23 00:00:00'),(139,'John','Seo','JSEO','650.121.2019','ST_CLERK',2700.00,NULL,123,50,'2002-12-23 00:00:00'),(140,'Joshua','Patel','JPATEL','650.121.1834','ST_CLERK',2500.00,NULL,123,50,'2002-12-23 00:00:00'),(141,'Trenna','Rajs','TRAJS','650.121.8009','ST_CLERK',3500.00,NULL,124,50,'2002-12-23 00:00:00'),(142,'Curtis','Davies','CDAVIES','650.121.2994','ST_CLERK',3100.00,NULL,124,50,'2002-12-23 00:00:00'),(143,'Randall','Matos','RMATOS','650.121.2874','ST_CLERK',2600.00,NULL,124,50,'2002-12-23 00:00:00'),(144,'Peter','Vargas','PVARGAS','650.121.2004','ST_CLERK',2500.00,NULL,124,50,'2002-12-23 00:00:00'),(145,'John','Russell','JRUSSEL','011.44.1344.429268','SA_MAN',14000.00,0.40,100,80,'2002-12-23 00:00:00'),(146,'Karen','Partners','KPARTNER','011.44.1344.467268','SA_MAN',13500.00,0.30,100,80,'2002-12-23 00:00:00'),(147,'Alberto','Errazuriz','AERRAZUR','011.44.1344.429278','SA_MAN',12000.00,0.30,100,80,'2002-12-23 00:00:00'),(148,'Gerald','Cambrault','GCAMBRAU','011.44.1344.619268','SA_MAN',11000.00,0.30,100,80,'2002-12-23 00:00:00'),(149,'Eleni','Zlotkey','EZLOTKEY','011.44.1344.429018','SA_MAN',10500.00,0.20,100,80,'2002-12-23 00:00:00'),(150,'Peter','Tucker','PTUCKER','011.44.1344.129268','SA_REP',10000.00,0.30,145,80,'2014-03-05 00:00:00'),(151,'David','Bernstein','DBERNSTE','011.44.1344.345268','SA_REP',9500.00,0.25,145,80,'2014-03-05 00:00:00'),(152,'Peter','Hall','PHALL','011.44.1344.478968','SA_REP',9000.00,0.25,145,80,'2014-03-05 00:00:00'),(153,'Christopher','Olsen','COLSEN','011.44.1344.498718','SA_REP',8000.00,0.20,145,80,'2014-03-05 00:00:00'),(154,'Nanette','Cambrault','NCAMBRAU','011.44.1344.987668','SA_REP',7500.00,0.20,145,80,'2014-03-05 00:00:00'),(155,'Oliver','Tuvault','OTUVAULT','011.44.1344.486508','SA_REP',7000.00,0.15,145,80,'2014-03-05 00:00:00'),(156,'Janette','K_ing','JKING','011.44.1345.429268','SA_REP',10000.00,0.35,146,80,'2014-03-05 00:00:00'),(157,'Patrick','Sully','PSULLY','011.44.1345.929268','SA_REP',9500.00,0.35,146,80,'2014-03-05 00:00:00'),(158,'Allan','McEwen','AMCEWEN','011.44.1345.829268','SA_REP',9000.00,0.35,146,80,'2014-03-05 00:00:00'),(159,'Lindsey','Smith','LSMITH','011.44.1345.729268','SA_REP',8000.00,0.30,146,80,'2014-03-05 00:00:00'),(160,'Louise','Doran','LDORAN','011.44.1345.629268','SA_REP',7500.00,0.30,146,80,'2014-03-05 00:00:00'),(161,'Sarath','Sewall','SSEWALL','011.44.1345.529268','SA_REP',7000.00,0.25,146,80,'2014-03-05 00:00:00'),(162,'Clara','Vishney','CVISHNEY','011.44.1346.129268','SA_REP',10500.00,0.25,147,80,'2014-03-05 00:00:00'),(163,'Danielle','Greene','DGREENE','011.44.1346.229268','SA_REP',9500.00,0.15,147,80,'2014-03-05 00:00:00'),(164,'Mattea','Marvins','MMARVINS','011.44.1346.329268','SA_REP',7200.00,0.10,147,80,'2014-03-05 00:00:00'),(165,'David','Lee','DLEE','011.44.1346.529268','SA_REP',6800.00,0.10,147,80,'2014-03-05 00:00:00'),(166,'Sundar','Ande','SANDE','011.44.1346.629268','SA_REP',6400.00,0.10,147,80,'2014-03-05 00:00:00'),(167,'Amit','Banda','ABANDA','011.44.1346.729268','SA_REP',6200.00,0.10,147,80,'2014-03-05 00:00:00'),(168,'Lisa','Ozer','LOZER','011.44.1343.929268','SA_REP',11500.00,0.25,148,80,'2014-03-05 00:00:00'),(169,'Harrison','Bloom','HBLOOM','011.44.1343.829268','SA_REP',10000.00,0.20,148,80,'2014-03-05 00:00:00'),(170,'Tayler','Fox','TFOX','011.44.1343.729268','SA_REP',9600.00,0.20,148,80,'2014-03-05 00:00:00'),(171,'William','Smith','WSMITH','011.44.1343.629268','SA_REP',7400.00,0.15,148,80,'2014-03-05 00:00:00'),(172,'Elizabeth','Bates','EBATES','011.44.1343.529268','SA_REP',7300.00,0.15,148,80,'2014-03-05 00:00:00'),(173,'Sundita','Kumar','SKUMAR','011.44.1343.329268','SA_REP',6100.00,0.10,148,80,'2014-03-05 00:00:00'),(174,'Ellen','Abel','EABEL','011.44.1644.429267','SA_REP',11000.00,0.30,149,80,'2014-03-05 00:00:00'),(175,'Alyssa','Hutton','AHUTTON','011.44.1644.429266','SA_REP',8800.00,0.25,149,80,'2014-03-05 00:00:00'),(176,'Jonathon','Taylor','JTAYLOR','011.44.1644.429265','SA_REP',8600.00,0.20,149,80,'2014-03-05 00:00:00'),(177,'Jack','Livingston','JLIVINGS','011.44.1644.429264','SA_REP',8400.00,0.20,149,80,'2014-03-05 00:00:00'),(178,'Kimberely','Grant','KGRANT','011.44.1644.429263','SA_REP',7000.00,0.15,149,NULL,'2014-03-05 00:00:00'),(179,'Charles','Johnson','CJOHNSON','011.44.1644.429262','SA_REP',6200.00,0.10,149,80,'2014-03-05 00:00:00'),(180,'Winston','Taylor','WTAYLOR','650.507.9876','SH_CLERK',3200.00,NULL,120,50,'2014-03-05 00:00:00'),(181,'Jean','Fleaur','JFLEAUR','650.507.9877','SH_CLERK',3100.00,NULL,120,50,'2014-03-05 00:00:00'),(182,'Martha','Sullivan','MSULLIVA','650.507.9878','SH_CLERK',2500.00,NULL,120,50,'2014-03-05 00:00:00'),(183,'Girard','Geoni','GGEONI','650.507.9879','SH_CLERK',2800.00,NULL,120,50,'2014-03-05 00:00:00'),(184,'Nandita','Sarchand','NSARCHAN','650.509.1876','SH_CLERK',4200.00,NULL,121,50,'2014-03-05 00:00:00'),(185,'Alexis','Bull','ABULL','650.509.2876','SH_CLERK',4100.00,NULL,121,50,'2014-03-05 00:00:00'),(186,'Julia','Dellinger','JDELLING','650.509.3876','SH_CLERK',3400.00,NULL,121,50,'2014-03-05 00:00:00'),(187,'Anthony','Cabrio','ACABRIO','650.509.4876','SH_CLERK',3000.00,NULL,121,50,'2014-03-05 00:00:00'),(188,'Kelly','Chung','KCHUNG','650.505.1876','SH_CLERK',3800.00,NULL,122,50,'2014-03-05 00:00:00'),(189,'Jennifer','Dilly','JDILLY','650.505.2876','SH_CLERK',3600.00,NULL,122,50,'2014-03-05 00:00:00'),(190,'Timothy','Gates','TGATES','650.505.3876','SH_CLERK',2900.00,NULL,122,50,'2014-03-05 00:00:00'),(191,'Randall','Perkins','RPERKINS','650.505.4876','SH_CLERK',2500.00,NULL,122,50,'2014-03-05 00:00:00'),(192,'Sarah','Bell','SBELL','650.501.1876','SH_CLERK',4000.00,NULL,123,50,'2014-03-05 00:00:00'),(193,'Britney','Everett','BEVERETT','650.501.2876','SH_CLERK',3900.00,NULL,123,50,'2014-03-05 00:00:00'),(194,'Samuel','McCain','SMCCAIN','650.501.3876','SH_CLERK',3200.00,NULL,123,50,'2014-03-05 00:00:00'),(195,'Vance','Jones','VJONES','650.501.4876','SH_CLERK',2800.00,NULL,123,50,'2014-03-05 00:00:00'),(196,'Alana','Walsh','AWALSH','650.507.9811','SH_CLERK',3100.00,NULL,124,50,'2014-03-05 00:00:00'),(197,'Kevin','Feeney','KFEENEY','650.507.9822','SH_CLERK',3000.00,NULL,124,50,'2014-03-05 00:00:00'),(198,'Donald','OConnell','DOCONNEL','650.507.9833','SH_CLERK',2600.00,NULL,124,50,'2014-03-05 00:00:00'),(199,'Douglas','Grant','DGRANT','650.507.9844','SH_CLERK',2600.00,NULL,124,50,'2014-03-05 00:00:00'),(200,'Jennifer','Whalen','JWHALEN','515.123.4444','AD_ASST',4400.00,NULL,101,10,'2016-03-03 00:00:00'),(201,'Michael','Hartstein','MHARTSTE','515.123.5555','MK_MAN',13000.00,NULL,100,20,'2016-03-03 00:00:00'),(202,'Pat','Fay','PFAY','603.123.6666','MK_REP',6000.00,NULL,201,20,'2016-03-03 00:00:00'),(203,'Susan','Mavris','SMAVRIS','515.123.7777','HR_REP',6500.00,NULL,101,40,'2016-03-03 00:00:00'),(204,'Hermann','Baer','HBAER','515.123.8888','PR_REP',10000.00,NULL,101,70,'2016-03-03 00:00:00'),(205,'Shelley','Higgins','SHIGGINS','515.123.8080','AC_MGR',12000.00,NULL,101,110,'2016-03-03 00:00:00'),(206,'William','Gietz','WGIETZ','515.123.8181','AC_ACCOUNT',8300.00,NULL,205,110,'2016-03-03 00:00:00');
/*职位信息表*/
DROP TABLE IF EXISTS `jobs`;
CREATE TABLE `jobs` (
`job_id` varchar(10) NOT NULL comment '职位id',
`job_title` varchar(35) DEFAULT NULL comment '职位名称',
`min_salary` int(6) DEFAULT NULL comment '薪资范围最小值',
`max_salary` int(6) DEFAULT NULL comment '薪资范围最大值',
PRIMARY KEY (`job_id`)
) ENGINE=InnoDB comment '职位id';
insert into `jobs`(`job_id`,`job_title`,`min_salary`,`max_salary`) values ('AC_ACCOUNT','Public Accountant',4200,9000),('AC_MGR','Accounting Manager',8200,16000),('AD_ASST','Administration Assistant',3000,6000),('AD_PRES','President',20000,40000),('AD_VP','Administration Vice President',15000,30000),('FI_ACCOUNT','Accountant',4200,9000),('FI_MGR','Finance Manager',8200,16000),('HR_REP','Human Resources Representative',4000,9000),('IT_PROG','Programmer',4000,10000),('MK_MAN','Marketing Manager',9000,15000),('MK_REP','Marketing Representative',4000,9000),('PR_REP','Public Relations Representative',4500,10500),('PU_CLERK','Purchasing Clerk',2500,5500),('PU_MAN','Purchasing Manager',8000,15000),('SA_MAN','Sales Manager',10000,20000),('SA_REP','Sales Representative',6000,12000),('SH_CLERK','Shipping Clerk',2500,5500),('ST_CLERK','Stock Clerk',2000,5000),('ST_MAN','Stock Manager',5500,8500);
/*位置表*/
DROP TABLE IF EXISTS `locations`;
CREATE TABLE `locations` (
`location_id` int(11) NOT NULL AUTO_INCREMENT comment '位置id',
`street_address` varchar(40) DEFAULT NULL comment '街道地址',
`postal_code` varchar(12) DEFAULT NULL comment '邮编',
`city` varchar(30) DEFAULT NULL comment '城市名称',
`state_province` varchar(25) DEFAULT NULL comment '省',
`country_id` varchar(2) DEFAULT NULL comment '国家编号',
PRIMARY KEY (`location_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3201 comment '位置表';
insert into `locations`(`location_id`,`street_address`,`postal_code`,`city`,`state_province`,`country_id`) values (1000,'1297 Via Cola di Rie','00989','Roma',NULL,'IT'),(1100,'93091 Calle della Testa','10934','Venice',NULL,'IT'),(1200,'2017 Shinjuku-ku','1689','Tokyo','Tokyo Prefecture','JP'),(1300,'9450 Kamiya-cho','6823','Hiroshima',NULL,'JP'),(1400,'2014 Jabberwocky Rd','26192','Southlake','Texas','US'),(1500,'2011 Interiors Blvd','99236','South San Francisco','California','US'),(1600,'2007 Zagora St','50090','South Brunswick','New Jersey','US'),(1700,'2004 Charade Rd','98199','Seattle','Washington','US'),(1800,'147 Spadina Ave','M5V 2L7','Toronto','Ontario','CA'),(1900,'6092 Boxwood St','YSW 9T2','Whitehorse','Yukon','CA'),(2000,'40-5-12 Laogianggen','190518','Beijing',NULL,'CN'),(2100,'1298 Vileparle (E)','490231','Bombay','Maharashtra','IN'),(2200,'12-98 Victoria Street','2901','Sydney','New South Wales','AU'),(2300,'198 Clementi North','540198','Singapore',NULL,'SG'),(2400,'8204 Arthur St',NULL,'London',NULL,'UK'),(2500,'Magdalen Centre, The Oxford Science Park','OX9 9ZB','Oxford','Oxford','UK'),(2600,'9702 Chester Road','09629850293','Stretford','Manchester','UK'),(2700,'Schwanthalerstr. 7031','80925','Munich','Bavaria','DE'),(2800,'Rua Frei Caneca 1360 ','01307-002','Sao Paulo','Sao Paulo','BR'),(2900,'20 Rue des Corps-Saints','1730','Geneva','Geneve','CH'),(3000,'Murtenstrasse 921','3095','Bern','BE','CH'),(3100,'Pieter Breughelstraat 837','3029SK','Utrecht','Utrecht','NL'),(3200,'Mariano Escobedo 9991','11932','Mexico City','Distrito Federal,','MX');
/*薪资等级表*/
DROP TABLE IF EXISTS `job_grades`;
CREATE TABLE `job_grades`(
`grade_level` varchar(3) comment '等级',
`lowest_sal` int comment '薪资最低值',
`highest_sal` int comment '薪资最高值',
PRIMARY KEY (`grade_level`)
) comment '薪资等级表';
INSERT INTO job_grades VALUES ('A', 1000, 2999),('B', 3000, 5999),('C', 6000, 9999),('D', 10000, 14999),('E', 15000, 24999),('F', 25000, 40000);
select后面的子查询
子查询位于select后面的，仅仅支持标量子查询。
示例1
查询每个部门员工人数
select
a.*,
(SELECT count(*) from employees b where b.department_id = a.department_id) as 员工人数
from departments a;
示例2
查询员工号=102的部门名称
select (
select a.department_name
from departments a,employees b
where a.department_id = b.department_id and b.employee_id = 102
)
as 部门名称
from后面的子查询
将子查询的结果集充当一张表，要求必须起别名，否者这个表找不到。
然后将真实的表和子查询结果表进行连接查询。
示例1
查询每个部门平均工资的薪资等级
-- 查询每个部门的平均工资
SELECT department_id,AVG(a.salary)
FROM employees a
GROUP BY a.department_id
-- 薪资等级表
SELECT * from job_grades;
-- 将上面2个结果连接查询，筛选条件:平均工资 between lowest_sal and highest_sal;
SELECT
t1.department_id,
sa AS '平均工资',
t2.grade_level
FROM (SELECT department_id,AVG(a.salary) sa
FROM employees a
GROUP BY a.department_id
) t1, job_grades t2
WHERE
t1.sa BETWEEN t2.lowest_sal AND t2.highest_sal;
执行结果：
+---------------+--------------+-------------+
| department_id | 平均工资 | grade_level |
+---------------+--------------+-------------+
| NULL | 7000.000000 | C |
| 10 | 4400.000000 | B |
| 20 | 9500.000000 | C |
| 30 | 4150.000000 | B |
| 40 | 6500.000000 | C |
| 50 | 3475.555556 | B |
| 60 | 5760.000000 | B |
| 70 | 10000.000000 | D |
| 80 | 8955.882353 | C |
| 90 | 19333.333333 | E |
| 100 | 8600.000000 | C |
| 110 | 10150.000000 | D |
+---------------+--------------+-------------+
12 rows in set (0.00 sec)
where和having后面的子查询
where或having后面，可以使用
1.标量子查询（单行单列行子查询）
2.列子查询（单列多行子查询）
3.行子查询（一行多列）
一些特点
- 子查询放在小括号内
- 子查询一般放在条件的右侧
- 标量子查询，一般搭配着单行单列相关的操作符使用>、<、>=、<=、<>、!=
- 列子查询，一般搭配多行操作符使用
in(not in)：列表中的“任意一个”
any或some(some是any的别名)：和子查询返回的“某一个值”比较，比如a>some(10,20,30)，a大于子查询中
任意一个即可，a大于子查询中最小值即可，等同于a>min(10,20,30)
all：和子查询返回的“所有值”比较，比如a>all(10,20,30)，a大于子查询中所有值，换句话说，a大于子查询中最大值即可满足查询条件，等同于a>max(10,20,30);
- 子查询的执行优先于主查询执行，因为主查询的条件用到了子查询的结果。
in、any、some、all
in，any，some，all分别是子查询关键词之一。
in：in常用于where表达式中，其作用是查询某个范围内的数据
any和some一样： 可以与=、>、>=、<、<=、<>结合起来使用，分别表示等于、大于、大于等于、小于、小于等于、不等于其中的任何一个数据。
all：可以与=、>、>=、<、<=、<>结合是来使用，分别表示等于、大于、大于等于、小于、小于等于、不等于其中的其中的所有数据。
标量子查询
一般标量子查询，示例
查询谁的工资比Abel高
/*首先查出Abel的工资*/
SELECT salary from employees where last_name = 'Abel'
/*查询员工信息表，查找salary满足大于Abel工资的结果*/
SELECT *
FROM employees a
WHERE a.salary > (SELECT salary from employees where last_name = 'Abel');
多个标量子查询，示例
返回job_id与141号员工相同，salary比143号员工多的员工、姓名、job_id和工资
/*查询141号员工的job_id*/
SELECT job_id from employees where employee_id = 141;
/*查询143号员工的salary*/
SELECT salary from employees where employee_id = 143;
/*查询员工的姓名、job_id和工资，要求与141号员工的job_id相等，比143号员工的工资高*/
SELECT
a.last_name 姓名,
a.job_id,
a.salary 工资
FROM employees a
WHERE
a.job_id = (SELECT job_id FROM employees WHERE employee_id = 141) AND
a.salary > (SELECT salary FROM employees WHERE employee_id = 143);
子查询加分组查询，示例
查询最低工资大于50号部门最低工资的部门id和其最低工资【having】
/*查询最低工资大于50号部门最低工资的部门id和其最低工资【having】*/
/*①查询50号部门的最低工资*/
SELECT MIN(salary) FROM employees where department_id = 50
/*②查询每个部门的最低工资*/
SELECT MIN(salary),department_id FROM employees GROUP BY department_id
/* ③在②的基础上筛选，满足min(salary)>① */
SELECT
MIN(a.salary) minsalary,
department_id
FROM employees a
GROUP BY a.department_id
HAVING MIN(a.salary) > (SELECT MIN(salary) FROM employees where department_id = 50)
列子查询
列子查询结果集一列多行
列子查询需要搭配多行操作符使用：in(not in)、any/some、all
为了提升效率，最好去重一下distinct关键字
示例1
返回location_id是1400或1700的部门中的所有员工姓名
/*返回location_id是1400或1700的部门中的所有员工姓名*/
/*方式1*/
/*①先从depatments表中查出location_id是1400或1700的部门编号*/
SELECT DISTINCT department_id from departments where location_id IN (1400,1700);
/*查询员工姓名，要求其中员工department_id在①中*/
SELECT a.last_name
FROM employees a
WHERE a.department_id IN
(SELECT DISTINCT department_id from departments where location_id IN (1400,1700))
/*方式2：使用any实现(注意符号)*/
SELECT a.last_name
FROM employees a
WHERE a.department_id = ANY
(SELECT DISTINCT department_id from departments where location_id IN (1400,1700))
/*<> ALL等同于 not in*/
SELECT a.last_name
FROM employees a
WHERE a.department_id <> ALL
(SELECT DISTINCT department_id from departments where location_id IN (1400,1700))
示例2
返回其他工种中比job_id为'IT_PROG'工种任意工资低的员工的员工号、姓名、job_id、salary
/*1.查询出job_id为IT_ROG工种的所有工资*/
SELECT salary FROM employees WHERE job_id = 'IT_PROG'
/*也就是说要小于上面查询出的工资列表中的任意一个*/
/*查询员工的员工号、姓名、job_id、salary，且salary小于1中的任意一个*/
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < ANY (SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG')
/*或者说salary小于1表salary的最大值*/
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees WHERE job_id = 'IT_PROG')
示例3
返回其他工种中比job_id为'IT_PROG'部门所有工资低的员工的员工号、姓名、job_id、salary
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < All (SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG') AND job_id != 'IT_PROG';
/*或*/
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < (SELECT MIN(salary) FROM employees WHERE job_id = 'IT_PROG') AND job_id != 'IT_PROG';
行子查询
子查询结果集为一行多列
示例
查询员工编号最小并且工资最高的员工信息，3种方式。
/*1.先查询出最小的员工编号*/
SELECT MIN(employee_id) FROM employees
/*2.查询工资最高的员工信息*/
SELECT MAX(salary)from employees
-- 方法1
SELECT *
FROM employees a
WHERE a.employee_id = (SELECT MIN(employee_id) FROM employees)
AND salary = (SELECT MAX(salary)from employees)
-- 方法2
SELECT *
FROM employees a
WHERE (a.employee_id,a.salary) = (SELECT MIN(employee_id),MAX(salary) FROM employees)
-- 方法3
SELECT *
FROM employees a
WHERE (a.employee_id,a.salary) IN (SELECT MIN(employee_id),MAX(salary) FROM employees)
exists后面（也叫相关子查询）
exists|not exists
当exists在where后面时，用于判断子查询的结果集是否为空，若子查询的结果集不为空，返回 TRUE，否则返回 FALSE；若使用关键字 NOT，则返回的值正好相反。
- 语法：exists(完整的查询语句)
- exists查询结果：1或0，exists的结果用来判断
- 一般来说，能用exists的子查询，绝对都能用in代替，所以exists用的少
- 和前面的查询不同，这先执行主查询，然后主查询查询的结果，在根据子查询进行过滤，子查询中涉及到主查询中用到的字段，所以叫相关子查询。
示例1
SELECT
EXISTS(SELECT employee_id FROM employees WHERE salary = 300000)
as 'exists返回1或者0';
+----------------------+
| exists返回1或者0 |
+----------------------+
| 0 |
+----------------------+
1 row in set (0.00 sec)
示例2
查询所有员工的部门名称
SELECT department_name
FROM departments a
WHERE EXISTS(SELECT 1 FROM employees b WHERE a.department_id = b.department_id)
/*用in实现*/
SELECT department_name
FROM departments a
WHERE a.department_id IN (SELECT department_id FROM employees);
示例3
查询没有员工的部门
-- exists实现
SELECT *
FROM departments a
WHERE NOT exists(SELECT 1 FROM employees b WHERE a.department_id = b.department_id AND b.department_id IS NOT NULL);
-- in实现
SELECT *
FROM departments a
WHERE a.department_id NOT IN (SELECT department_id FROM employees b WHERE b.department_id IS NOT NULL);
b.department_id IS NOT NULL为什么要用is not null
注意null值
MySQL 提供了 IS NULL 关键字，用来判断字段的值是否为空值（NULL）
空值不同于 0，也不同于空字符串
如果字段的值是空值，则满足查询条件，该记录将被查询出来。如果字段的值不是空值，则不满足查询条件。
注意：IS NULL 是一个整体，不能将 IS 换成“=”。如果将 IS 换成“=”将不能查询出任何结果，数据库系统会出现“Empty set(0.00 sec)”这样的提示。同理，IS NOT NULL 中的 IS NOT 不能换成“!=”或“<>”。
例如下面的例子，在使用not in方式查询没有员工的部门时，因为子查询的结果中包含了null值，导致外查询的结果为空
/*使用not in查询没有员工的部门*/
SELECT * FROM departments a WHERE a.department_id NOT IN (SELECT department_id FROM employees b);
-- 查询结果
mysql> SELECT * FROM departments a WHERE a.department_id NOT IN (SELECT department_id FROM employees b);
Empty set (0.00 sec)
not in的情况下，子查询中列的值为NULL的时候，外查询的结果为空。
小结：
在MySQL中，NULL值意味着未知值。 NULL值不是零或空字符”值。
NULL值不等于其自身值。 如果将NULL值与另一个NULL值或任何其他值进行比较，则结果为NULL，因为每个NULL值的值都是未知的。
通常，使用NULL值来表示数据丢失，未知或不适用。
当创建表时，可以通过使用NOT NULL约束来指定列是否接受NULL值。
NULL在相关排序时类似无穷小的值
所以，不建议使用 NULL 作为列默认值
