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MySQL 子查询使用方式

發(fā)布時間：2023/12/14 数据库 24 豆豆

生活随笔收集整理的這篇文章主要介紹了 MySQL 子查询使用方式小編覺得挺不錯的,現(xiàn)在分享給大家,幫大家做個參考.

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闡述
- 子查詢分類
- 按照返回結(jié)果的行列數(shù)分類
- 按子查詢出現(xiàn)在主查詢的位置分類
預(yù)備數(shù)據(jù)
- 部門表 departments
- 員工表 employees
- 職位信息表 jobs
- 位置表 locations
- 薪資等級表 job_grades
查詢數(shù)據(jù)示例
- select 后面的子查詢
- - 示例1 查詢每個部門員工人數(shù)
  - 示例2 查詢員工號 =102 的部門名稱
- from 后面的子查詢
- - 示例1 查詢每個部門平均工資的薪資等級
- where 和 having 后面的子查詢
標量子查詢
- 一般標量子查詢，查詢誰的工資比 Abel 高
- 多個標量子查詢，返回 job_id 與 141 號員工相同，salary 比143 號員工多的員工、姓名、job_id 和工資
- 子查詢加分組查詢，查詢最低工資大于 50 號部門最低工資的部門 id 和其最低工資【having】
列子查詢
- 示例1，返回 location_id 是 1400 或 1700 的部門中的所有員工姓名
- - 方式 1
  - 方式 2：使用 any 實現(xiàn) (注意符號)
  - <> ALL 等同于 not in
- 示例2，返回其他工種中比 job_id 為 'IT_PROG' 工種任意工資低的員工的員工號、姓名、job_id、salary
- 示例3，返回其他工種中比 job_id 為 'IT_PROG' 部門所有工資低的員工的員工號、姓名、job_id、salary
行子查詢
- 方法 1
- 方法 2
- 方法 3
exists 后面（也叫相關(guān)子查詢）
- 示例1
- 示例2，查詢所有員工的部門名稱
- 示例3，查詢沒有員工的部門
注意 null 值
- 小結(jié)

闡述

所謂子查詢，就是嵌套在其他查詢中的查詢。

某些情況下，當進行一個查詢時，需要的條件或數(shù)據(jù)要用另外一個 select 語句的結(jié)果，這個時候，就要用到子查詢。

定義：
執(zhí)行順序方面先于當前查詢執(zhí)行的，并且是嵌套在當前查詢中的查詢叫做子查詢。

MySQL 在處理上例的 SELECT 語句時，執(zhí)行流程為：

先執(zhí)行子查詢，再執(zhí)行父查詢。
子查詢應(yīng)當始終放在括號內(nèi)。

子查詢的支持是從 4.1 版本引入的。MySQL 的早期版本不支持子查詢。

子查詢分類

按照返回結(jié)果的行列數(shù)分類

1 標量子查詢（結(jié)果集只有一行一列） 2 列子查詢（結(jié)果集只有一列多行） 3 行子查詢（結(jié)果集有一行多列） 4 表子查詢（結(jié)果集一般為多列多行）

按子查詢出現(xiàn)在主查詢的位置分類

select 后面：僅支持標量子查詢from 后面：支持表子查詢where 或者 having 后面：支持標量子查詢（單行單列）、列子查詢（單列多行）、行子查詢（多列多行）exists后面（即相關(guān)子查詢）：表子查詢（多行多列）

預(yù)備數(shù)據(jù)

部門表 departments

/*部門表*/ DROP TABLE IF EXISTS `departments`; CREATE TABLE `departments` (`department_id` int(4) NOT NULL AUTO_INCREMENT comment '部門id',`department_name` varchar(3) DEFAULT NULL comment '部門名稱',`manager_id` int(6) DEFAULT NULL comment '管理者id',`location_id` int(4) DEFAULT NULL comment '部門位置id，來源于表locations中的location_id',PRIMARY KEY (`department_id`),KEY `loc_id_fk` (`location_id`) ) ENGINE=InnoDB AUTO_INCREMENT=271 comment '部門表';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);

員工表 employees

/*員工表*/ DROP TABLE IF EXISTS `employees`; CREATE TABLE `employees` (`employee_id` int(6) NOT NULL AUTO_INCREMENT comment '員工id',`first_name` varchar(20) DEFAULT NULL comment '名',`last_name` varchar(25) DEFAULT NULL comment '姓',`email` varchar(25) DEFAULT NULL comment '電子郵箱',`phone_number` varchar(20) DEFAULT NULL comment '手機',`job_id` varchar(10) DEFAULT NULL comment '職位id，來源于jobs表中的job_id',`salary` double(10,2) DEFAULT NULL comment '薪水',`commission_pct` double(4,2) DEFAULT NULL comment '傭金百分比',`manager_id` int(6) DEFAULT NULL comment '上級id',`department_id` int(4) DEFAULT NULL comment '所屬部門id，來源于departments中的department_id',`hiredate` datetime DEFAULT NULL comment '入職日期',PRIMARY KEY (`employee_id`) ) ENGINE=InnoDB AUTO_INCREMENT=207 comment '員工表';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');

職位信息表 jobs

/*職位信息表*/ DROP TABLE IF EXISTS `jobs`; CREATE TABLE `jobs` (`job_id` varchar(10) NOT NULL comment '職位id',`job_title` varchar(35) DEFAULT NULL comment '職位名稱',`min_salary` int(6) DEFAULT NULL comment '薪資范圍最小值',`max_salary` int(6) DEFAULT NULL comment '薪資范圍最大值',PRIMARY KEY (`job_id`) ) ENGINE=InnoDB comment '職位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 locations

/*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 '郵編',`city` varchar(30) DEFAULT NULL comment '城市名稱',`state_province` varchar(25) DEFAULT NULL comment '省',`country_id` varchar(2) DEFAULT NULL comment '國家編號',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');

薪資等級表 job_grades

/*薪資等級表*/ DROP TABLE IF EXISTS `job_grades`; CREATE TABLE `job_grades`(`grade_level` varchar(3) comment '等級',`lowest_sal` int comment '薪資最低值',`highest_sal` int comment '薪資最高值',PRIMARY KEY (`grade_level`) ) comment '薪資等級表'; INSERT INTO job_grades VALUES('A', 1000, 2999),('B', 3000, 5999),('C', 6000, 9999),('D', 10000, 14999),('E', 15000, 24999),('F', 25000, 40000);

查詢數(shù)據(jù)示例

1 標量子查詢（結(jié)果集只有一行一列） 2 列子查詢（結(jié)果集只有一列多行） 3 行子查詢（結(jié)果集有一行多列） 4 表子查詢（結(jié)果集一般為多列多行） select 后面：僅支持標量子查詢from 后面：支持表子查詢where 或者 having 后面：支持標量子查詢（單行單列）、列子查詢（單列多行）、行子查詢（多列多行）exists后面（即相關(guān)子查詢）：表子查詢（多行多列）

select 后面的子查詢

子查詢位于 select 后面的，僅僅支持標量子查詢。

示例1 查詢每個部門員工人數(shù)

SELECTa.*, (SELECTcount(*)FROMemployees bWHEREb.department_id = a.department_id) AS 員工人數(shù) FROMdepartments a;

示例2 查詢員工號 =102 的部門名稱

select (select a.department_namefrom departments a,employees b where a.department_id = b.department_id and b.employee_id = 102) as 部門名稱

from 后面的子查詢

將子查詢的結(jié)果集充當一張表，要求必須起別名，否者這個表找不到。

然后將真實的表和子查詢結(jié)果表進行連接查詢。

示例1 查詢每個部門平均工資的薪資等級

-- 查詢每個部門的平均工資 SELECT department_id,AVG(a.salary) FROM employees a GROUP BY a.department_id-- 薪資等級表 SELECT * from job_grades;-- 將上面2個結(jié)果連接查詢，篩選條件:平均工資 between lowest_sal and highest_sal;SELECT t1.department_id,sa AS '平均工資',t2.grade_level FROM (SELECT department_id,AVG(a.salary) saFROM employees aGROUP BY a.department_id) t1, job_grades t2 WHEREt1.sa BETWEEN t2.lowest_sal AND t2.highest_sal;

where 和 having 后面的子查詢

1.標量子查詢（單行單列行子查詢）

2.列子查詢（單列多行子查詢）

3.行子查詢（一行多列）

一些特點

1 子查詢放在小括號內(nèi)
2 子查詢一般放在條件的右側(cè)
3 標量子查詢，一般搭配著單行單列相關(guān)的操作符使用 >、<、>=、<=、<>、!=
4 列子查詢，一般搭配多行操作符使用

in(not in)：列表中的“任意一個”any 或 some (some 是 any 的別名)：和子查詢返回的“某一個值”比較，比如 a>some(10,20,30)， a 大于子查詢中任意一個即可，a 大于子查詢中最小值即可，等同于 a > min(10,20,30)all：和子查詢返回的“所有值”比較，比如 a > all(10,20,30)， a 大于子查詢中所有值，換句話說，a 大于子查詢中最大值即可滿足查詢條件，等同于 a > max(10,20,30);

5 子查詢的執(zhí)行優(yōu)先于主查詢執(zhí)行，因為主查詢的條件用到了子查詢的結(jié)果。

in、any、some、all

in，any，some，all 分別是子查詢關(guān)鍵詞之一。in：in 常用于 where 表達式中，其作用是查詢某個范圍內(nèi)的數(shù)據(jù)。any 和 some 一樣：可以與 =、>、>=、<、<=、<> 結(jié)合起來使用，分別表示等于、大于、大于等于、小于、小于等于、不等于其中的任何一個數(shù)據(jù)。all：可以與 =、>、>=、<、<=、<> 結(jié)合是來使用，分別表示等于、大于、大于等于、小于、小于等于、不等于其中的其中的所有數(shù)據(jù)。

標量子查詢

一般標量子查詢，查詢誰的工資比 Abel 高

/*首先查出 Abel 的工資*/ SELECT salary from employees where last_name = 'Abel'/*查詢員工信息表，查找 salary 滿足大于 Abel 工資的結(jié)果*/ SELECT* FROMemployees a WHEREa.salary > (SELECTsalaryFROMemployeesWHERElast_name = 'Abel');

多個標量子查詢，返回 job_id 與 141 號員工相同，salary 比143 號員工多的員工、姓名、job_id 和工資

/*查詢 141 號員工的 job_id */ SELECT job_id from employees where employee_id = 141;

/*查詢 143 號員工的 salary */ SELECT salary from employees where employee_id = 143;

/*查詢員工的姓名、job_id 和工資，要求與 141 號員工的 job_id 相等，比 143 號員工的工資高 */SELECTa.last_name 姓名,a.job_id,a.salary 工資 FROMemployees a WHEREa.job_id = (SELECTjob_idFROMemployeesWHEREemployee_id = 141) AND a.salary > (SELECTsalaryFROMemployeesWHEREemployee_id = 143 );

子查詢加分組查詢，查詢最低工資大于 50 號部門最低工資的部門 id 和其最低工資【having】

查詢最低工資大于 50 號部門最低工資的部門 id 和其最低工資【having】

① 查詢 50 號部門的最低工資

SELECT MIN(salary) FROM employees where department_id = 50

② 查詢每個部門的最低工資

SELECT MIN(salary),department_id FROM employees GROUP BY department_id

③ 在 ② 的基礎(chǔ)上篩選，滿足 min(salary) > ①

SELECTMIN(a.salary) minsalary,department_id FROMemployees a GROUP BYa.department_id HAVINGMIN(a.salary) > (SELECTMIN(salary)FROMemployeesWHEREdepartment_id = 50)

列子查詢

列子查詢結(jié)果集一列多行。

列子查詢需要搭配多行操作符使用：in(not in)、any/some、all

為了提升效率，最好去重一下 distinct 關(guān)鍵字。

示例1，返回 location_id 是 1400 或 1700 的部門中的所有員工姓名

方式 1

① 先從 depatments 表中查出 location_id 是 1400 或 1700 的部門編號

SELECT DISTINCTdepartment_id FROMdepartments WHERElocation_id IN (1400, 1700);

查詢員工姓名，要求其中員工 department_id 在 ① 中

SELECTa.last_name FROMemployees a WHEREa.department_id IN (SELECT DISTINCTdepartment_idFROMdepartmentsWHERElocation_id IN (1400, 1700))

方式 2：使用 any 實現(xiàn) (注意符號)

SELECTa.last_name FROMemployees a WHEREa.department_id = ANY (SELECT DISTINCTdepartment_idFROMdepartmentsWHERElocation_id IN (1400, 1700))

<> ALL 等同于 not in

SELECTa.last_name FROMemployees a WHEREa.department_id <> ALL (SELECT DISTINCTdepartment_idFROMdepartmentsWHERElocation_id IN (1400, 1700))

示例2，返回其他工種中比 job_id 為 ‘IT_PROG’ 工種任意工資低的員工的員工號、姓名、job_id、salary

1 查詢出 job_id 為 IT_ROG 工種的所有工資

SELECT salary FROM employees WHERE job_id = 'IT_PROG'

也就是說要小于上面查詢出的工資列表中的任意一個。

查詢員工的員工號、姓名、job_id、salary，且 salary 小于 1 中的任意一個。

SELECTlast_name,employee_id,job_id,salary FROMemployees WHEREsalary < ANY (SELECT DISTINCTsalaryFROMemployeesWHEREjob_id = 'IT_PROG')

或者說 salary 小于 1 表 salary 的最大值。

SELECTlast_name,employee_id,job_id,salary FROMemployees WHEREsalary < (SELECTMAX(salary)FROMemployeesWHEREjob_id = 'IT_PROG')

示例3，返回其他工種中比 job_id 為 ‘IT_PROG’ 部門所有工資低的員工的員工號、姓名、job_id、salary

SELECTlast_name,employee_id,job_id,salary FROMemployees WHEREsalary < ALL (SELECT DISTINCTsalaryFROMemployeesWHEREjob_id = 'IT_PROG') AND job_id != 'IT_PROG';

或

SELECTlast_name,employee_id,job_id,salary FROMemployees WHEREsalary < (SELECTMIN(salary)FROMemployeesWHEREjob_id = 'IT_PROG') AND job_id != 'IT_PROG';

查詢結(jié)果和上面一樣。

行子查詢

子查詢結(jié)果集為一行多列。

示例，查詢員工編號最小并且工資最高的員工信息，3 種方式。

1 先查詢出最小的員工編號

SELECT MIN(employee_id) FROM employees

2 查詢工資最高的員工信息

SELECT MAX(salary)from employees

方法 1

SELECT* FROMemployees a WHEREa.employee_id = (SELECTMIN(employee_id)FROMemployees) AND salary = (SELECTMAX(salary)FROMemployees )

方法 2

SELECT* FROMemployees a WHERE(a.employee_id, a.salary) = (SELECTMIN(employee_id),MAX(salary)FROMemployees)

方法 3

SELECT* FROMemployees a WHERE(a.employee_id, a.salary) IN (SELECTMIN(employee_id),MAX(salary)FROMemployees)

exists 后面（也叫相關(guān)子查詢）

exists | not exists

當 exists 在 where 后面時，
用于判斷子查詢的結(jié)果集是否為空，
若子查詢的結(jié)果集不為空，返回 TRUE，否則返回 FALSE；
若使用關(guān)鍵字 NOT，則返回的值正好相反。

1 語法：exists (完整的查詢語句)
2 exists 查詢結(jié)果：1 或 0，exists 的結(jié)果用來判斷
3 一般來說，能用 exists 的子查詢，絕對都能用 in 代替，所以 exists 用的少

4 和前面的查詢不同，這先執(zhí)行主查詢，然后主查詢查詢的結(jié)果，在根據(jù)子查詢進行過濾，子查詢中涉及到主查詢中用到的字段，所以叫相關(guān)子查詢。

示例1

SELECTEXISTS (SELECTemployee_idFROMemployeesWHEREsalary = 300000) AS 'exists返回1或者0';

示例2，查詢所有員工的部門名稱

SELECTdepartment_name FROMdepartments a WHEREEXISTS (SELECT1FROMemployees bWHEREa.department_id = b.department_id)

用 in 實現(xiàn)

SELECTdepartment_name FROMdepartments a WHEREa.department_id IN (SELECTdepartment_idFROMemployees);

示例3，查詢沒有員工的部門

exists 實現(xiàn)

SELECT* FROMdepartments a WHERENOT EXISTS (SELECT1FROMemployees bWHEREa.department_id = b.department_idAND b.department_id IS NOT NULL);

in 實現(xiàn)

SELECT* FROMdepartments a WHEREa.department_id NOT IN (SELECTdepartment_idFROMemployees bWHEREb.department_id IS NOT NULL);

注意 null 值

MySQL 提供了 IS NULL 關(guān)鍵字，用來判斷字段的值是否為空值（NULL）。

空值不同于 0，也不同于空字符串。

如果字段的值是空值，則滿足查詢條件，該記錄將被查詢出來。
如果字段的值不是空值，則不滿足查詢條件。

注意：

IS NULL 是一個整體，不能將 IS 換成 “=”。
如果將 IS 換成 “=” 將不能查詢出任何結(jié)果，數(shù)據(jù)庫系統(tǒng)會出現(xiàn) “Empty set(0.00 sec)” 這樣的提示。同理，IS NOT NULL 中的 IS NOT 不能換成 “!=” 或 “<>”。

例如下面的例子，在使用 not in 方式查詢沒有員工的部門時，
因為子查詢的結(jié)果中包含了 null 值，導(dǎo)致外查詢的結(jié)果為空。

使用 not in 查詢沒有員工的部門

SELECT* FROMdepartments a WHEREa.department_id NOT IN (SELECTdepartment_idFROMemployees b);

-- 查詢結(jié)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 的情況下，子查詢中列的值為 NULL 的時候，外查詢的結(jié)果為空。

小結(jié)

在 MySQL 中，NULL 值意味著未知值。
NULL 值不是零或空字符” 值。

NULL 值不等于其自身值。

如果將 NULL 值與另一個 NULL 值或任何其他值進行比較，則結(jié)果為 NULL，因為每個NULL 值的值都是未知的。

通常，使用 NULL 值來表示數(shù)據(jù)丟失，未知或不適用。

當創(chuàng)建表時，可以通過使用 NOT NULL 約束來指定列是否接受 NULL 值。

NULL 在相關(guān)排序時類似無窮小的值，所以，不建議使用 NULL 作為列默認值。

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