MySQL初級入門篇之檢視的相關概念及應用範例

2022-04-19 13:00:53

1.淺談資料庫物件

2.什麼是檢視？

檢視一方面可以幫我們使用表的一部分而不是所有的表，另一方面也可以針對不同的使用者制定不同的查詢檢視。比如，針對一個公司的銷售人員，我們只想給他看部分資料，而某些特殊的資料，比如採購的價格，則不會提供給他。再比如，人員薪酬是個敏感的欄位，那麼只給某個級別以上的人員開放，其他人的查詢檢視中則不提供這個欄位。
檢視是一種虛擬表，本身是不具有資料的，佔用很少的記憶體空間，它是 SQL 中的一個重要概念。
檢視建立在已有表的基礎上, 檢視賴以建立的這些表稱為基表。
檢視的建立和刪除隻影響檢視本身，不影響對應的基表。但是當對檢視中的資料進行增加、刪除和修改操作時，資料表中的資料會相應地發生變化，反之亦然。
檢視，是向用戶提供基表資料的另一種表現形式。通常情況下，小型專案的資料庫可以不使用視圖，但是在大型專案中，以及資料表比較複雜的情況下，檢視的價值就凸顯出來了，它可以幫助我們把經常查詢的結果集放到虛擬表中，提升使用效率。理解和使用起來都非常方便。

3.檢視操作相關SQL

建立檢視

CREATE [OR REPLACE] [ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}] VIEW 檢視名稱 [(欄位列表)]
AS
查詢語句 [WITH [CASCADED|LOCAL] CHECK OPTION]

檢視資料庫中的表物件、檢視物件

SHOW TABLES;

檢視檢視的結構

DESC / DESCRIBE 檢視名稱;

檢視檢視的屬性資訊

SHOW TABLE STATUS LIKE '檢視名稱'G

檢視檢視的詳細定義資訊

SHOW CREATE VIEW 檢視名稱;

關於更新檢視，要使檢視可更新，檢視中的行和底層基本表中的行之間必須存在一對一的關係。另外當檢視定義出現如下情況時，檢視不支援更新操作：
在定義檢視的時候指定了 “ALGORITHM = TEMPTABLE” ，檢視將不支援 INSERT 和 DELETE 操作；
檢視中不包含基表中所有被定義為非空又未指定預設值的列，檢視將不支援 INSERT 操作；
在定義檢視的 SELECT 語句中使用了 JOIN 聯合查詢，檢視將不支援 INSERT 和 DELETE 操作；
在定義檢視的 SELECT 語句後的欄位列表中使用了數學表示式或子查詢，檢視將不支援 INSERT ，也不支援 UPDATE 使用了數學表示式、子查詢的欄位值；
在定義檢視的 SELECT 語句後的欄位列表中使用 DISTINCT 、聚合函數、 GROUP BY 、 HAVING 、 UNION 等，檢視將不支援 INSERT 、 UPDATE 、 DELETE ；
在定義檢視的 SELECT 語句中包含了子查詢，而子查詢中參照了 FROM 後面的表，檢視將不支援 INSERT 、 UPDATE 、 DELETE ；
檢視定義基於一個不可更新檢視；
常數檢視。
雖然可以更新檢視資料，但總的來說，檢視作為虛擬表，主要用於方便查詢，不建議更新檢視的資料。 對檢視資料的更改，都是通過對實際資料表裡資料的操作來完成的。

使用 CREATE OR REPLACE VIEW 子句 修改檢視
刪除檢視只是刪除檢視的定義，並不會刪除基表的資料。
刪除檢視的語法是：（說明：基於檢視 a 、 b 建立了新的檢視 c ，如果將檢視 a 或者檢視 b 刪除，會導致檢視 c 的查詢失敗。這樣的檢視 c 需要手動刪除或修改，否則影響使用。）

DROP VIEW IF EXISTS 檢視名稱;

4.檢視實操SQL

首先，我們會基於employees表來建立新的檢視，下面先給出這個表的sql指令碼程式碼。

/*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);

首先，我們基於上表建立一個新的檢視。

CREATE TABLE emps
AS 
SELECT *
FROM employees;

建立檢視 emp_v1, 要求查詢電話號碼以 ‘011’ 開頭的員工姓名和工資、郵箱

CREATE OR REPLACE VIEW emp_v1
AS 
SELECT last_name, salary, email
FROM emps
WHERE phone_number LIKE '011%';

要求將檢視 emp_v1 修改為查詢電話號碼以 ‘011’ 開頭的並且郵箱中包含 e 字元的員工姓名和郵箱、電話號碼

CREATE OR REPLACE VIEW emp_v1
AS 
SELECT last_name, salary, email, phone_number
FROM emps
WHERE phone_number LIKE '011%' AND email LIKE '%e%';

向 emp_v1 插入一條記錄，是否可以？

DESC emps; 
DESC emp_v1; 
 
INSERT INTO emp_v1(last_name,salary,email,phone_number) 
VALUES('Tom',2300,'tom@126.com','1322321312'); 
 
#實測不可以

修改 emp_v1 中員工的工資，每人漲薪 1000

UPDATE emp_v1
SET salary = salary + 1000;
 
SELECT * FROM emp_v1;
SELECT * FROM emps;

刪除 emp_v1 中姓名為 Olsen 的員工

DELETE FROM emp_v1
WHERE last_name = 'Olsen';
 
SELECT *
FROM emp_v1
WHERE last_name = 'Olsen';

建立檢視 emp_v2 ，要求查詢部門的最高工資高於 12000 的部門 id 和其最高工資

CREATE OR REPLACE VIEW emp_v2
AS
SELECT department_id, MAX(salary) max_sal
FROM emps
GROUP BY department_id
HAVING MAX(salary) > 12000;
 
SELECT * FROM emp_v2;

向 emp_v2 中插入一條記錄，是否可以？

INSERT INTO emp_v2
VALUES(400, 18000);

刪除剛才的 emp_v2 和 emp_v1

DROP VIEW IF EXISTS emp_v1, emp_v2;
 
SHOW TABLES;

5.檢視的優缺點

1. 操作簡單

將經常使用的查詢操作定義為檢視，可以使開發人員不需要關心檢視對應的資料表的結構、表與表之間的關聯關係，也不需要關心資料表之間的業務邏輯和查詢條件，而只需要簡單地操作檢視即可，極大簡化了開發人員對資料庫的操作。

2. 減少資料冗餘

檢視跟實際資料表不一樣，它儲存的是查詢語句。所以，在使用的時候，我們要通過定義檢視的查詢語句來獲取結果集。而檢視本身不儲存資料，不佔用資料儲存的資源，減少了資料冗餘。

3. 資料安全

MySQL 將使用者對資料的存取限制在某些資料的結果集上，而這些資料的結果集可以使用檢視來實現。用戶不必直接查詢或運算元據表。這也可以理解為檢視具有隔離性。檢視相當於在使用者和實際的資料表之間加了一層虛擬表。同時， MySQL 可以根據許可權將使用者對資料的存取限制在某些檢視上，使用者不需要查詢資料表，可以直接通過檢視獲取資料表中的資訊。這在一定程度上保障了資料表中資料的安全性。

4. 適應靈活多變的需求當業務系統的需求發生變化後，如果需要改動資料表的結構，則工作量相對較大，可以使用檢視來減少改動的工作量。這種方式在實際工作中使用得比較多。

5. 能夠分解複雜的查詢邏輯資料庫中如果存在複雜的查詢邏輯，則可以將問題進行分解，建立多個檢視獲取資料，再將建立的多個檢視結合起來，完成複雜的查詢邏輯。

如果我們在實際資料表的基礎上建立了檢視，那麼，如果實際資料表的結構變更了，我們就需要及時對相關的檢視進行相應的維護。特別是巢狀的檢視（就是在檢視的基礎上建立檢視），維護會變得比較復雜，可讀性不好，容易變成系統的潛在隱患。因為建立檢視的 SQL 查詢可能會對欄位重新命名，也可能包含複雜的邏輯，這些都會增加維護的成本。

實際專案中，如果檢視過多，會導致資料庫維護成本的問題。

總結

到此這篇關於MySQL初級入門篇之檢視的相關概念及應用範例的文章就介紹到這了,更多相關MySQL檢視概念及應用內容請搜尋it145.com以前的文章或繼續瀏覽下面的相關文章希望大家以後多多支援it145.com！