view

Meaning: Understand as a virtual table and use it like a regular table
A new feature of MySQL version 5.1 is data generated dynamically from tables
For example: a comparison between a dance class and an ordinary class

Differences between views and tables:

Benefits of views:

1. sql statement improves reuse and efficiency
 2. Separation of tables to improve security

View Creation

Grammar:
CREATE VIEW View Name
AS
 Query statement;

View add-delete check

1,View the view's data ★

SELECT * FROM my_v4;
SELECT * FROM my_v1 WHERE last_name='Partners';

2,Insert data from view
INSERT INTO my_v4(last_name,department_id) VALUES('False Bamboo',90);

3,Modify the view's data

UPDATE my_v4 SET last_name ='Dream Girl' WHERE last_name='False Bamboo';


4,Delete data from view
DELETE FROM my_v4;

Some views cannot be updated

sql statements containing the following keywords: grouping function, distinct, group by, having, union, or union all
 Constant View
 Select contains subqueries
join
 from a view that cannot be updated
 A subquery of the where clause references a table in the from clause

Update of view logic

#Mode 1:
CREATE OR REPLACE VIEW test_v7
AS
SELECT last_name FROM employees
WHERE employee_id>100;

//Mode 2:
ALTER VIEW test_v7
AS
SELECT employee_id FROM employees;

SELECT * FROM test_v7;

Delete View

DROP VIEW test_v1,test_v2,test_v3;

View structure

DESC test_v7;
SHOW CREATE VIEW test_v7;

1. Create Views

Grammar:
create view view name
as
Query statement;

#Case: Query the student name and professional name of Zhang
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE 'Zhang%';

CREATE VIEW v1
AS
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`;

SELECT * FROM v1 WHERE stuname LIKE 'Zhang%';


#1. Create Views
/*
Grammar:
create view View Name
as
 Query statement;

*/
USE myemployees;

#1. Query employee name, department name and type information with a character in the name
#1. Create
CREATE VIEW myv1
AS

SELECT last_name,department_name,job_title
FROM employees e
JOIN departments d ON e.department_id  = d.department_id
JOIN jobs j ON j.job_id  = e.job_id;


#(2) Use
SELECT * FROM myv1 WHERE last_name LIKE '%a%';






#2. Query the average wage level of each department

#1. Create a view to see the average wage per department
CREATE VIEW myv2
AS
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id;

#(2) Use
SELECT myv2.`ag`,g.grade_level
FROM myv2
JOIN job_grades g
ON myv2.`ag` BETWEEN g.`lowest_sal` AND g.`highest_sal`;



#3. Query the Department with the lowest average wage

SELECT * FROM myv2 ORDER BY ag LIMIT 1;

#4. Query the name and salary of the Department with the lowest average wage

CREATE VIEW myv3
AS
SELECT * FROM myv2 ORDER BY ag LIMIT 1;


SELECT d.*,m.ag
FROM myv3 m
JOIN departments d
ON m.`department_id`=d.`department_id`;

2. Modification of Views

Mode 1:

Grammar:
create or replace view view name
as
Query statement;

Mode 2:

Grammar:
alter view view name
as
Query statement;

#2. Modification of Views

#Mode 1:
/*
create or replace view  View Name
as
 Query statement;

*/
SELECT * FROM myv3 

CREATE OR REPLACE VIEW myv3
AS
SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id;

#Mode 2:
/*
Grammar:
alter view View Name
as 
Query statement;

*/
ALTER VIEW myv3
AS
SELECT * FROM employees;

3. Delete View

Syntax: drop view view view name, view name,...;

#3. Delete View

/*

Syntax: drop view view view name, view name,...;
*/

DROP VIEW emp_v1,emp_v2,myv3;

4. View View

#4. View View

DESC myv3;

SHOW CREATE VIEW myv3;

5. Update of View

#5. Update of View

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email,salary*12*(1+IFNULL(commission_pct,0)) "annual salary"
FROM employees;

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
FROM employees;


SELECT * FROM myv1;
SELECT * FROM employees;
#1. Insert

INSERT INTO myv1 VALUES('Zhang Fei','zf@qq.com');

#2. Modification
UPDATE myv1 SET last_name = 'Zhang Wuji' WHERE last_name='Zhang Fei';

#3. Delete
DELETE FROM myv1 WHERE last_name = 'Zhang Wuji';

#Views with the following characteristics do not allow updates


#1. sql statements containing the following keywords: grouping function, distinct, group by, having, union, or union all

CREATE OR REPLACE VIEW myv1
AS
SELECT MAX(salary) m,department_id
FROM employees
GROUP BY department_id;

SELECT * FROM myv1;

#To update
UPDATE myv1 SET m=9000 WHERE department_id=10;

#(2) Constant View
CREATE OR REPLACE VIEW myv2
AS

SELECT 'john' NAME;

SELECT * FROM myv2;

#To update
UPDATE myv2 SET NAME='lucy';





#3. Select contains subqueries

CREATE OR REPLACE VIEW myv3
AS

SELECT department_id,(SELECT MAX(salary) FROM employees) Maximum Wage
FROM departments;

#To update
SELECT * FROM myv3;
UPDATE myv3 SET Maximum Wage=100000;


#④join
CREATE OR REPLACE VIEW myv4
AS

SELECT last_name,department_name
FROM employees e
JOIN departments d
ON e.department_id  = d.department_id;

#To update

SELECT * FROM myv4;
UPDATE myv4 SET last_name  = 'Zhang Fei' WHERE last_name='Whalen';
INSERT INTO myv4 VALUES('Chen Zhen','xxxx');



#from A view that cannot be updated
CREATE OR REPLACE VIEW myv5
AS

SELECT * FROM myv3;

#To update

SELECT * FROM myv5;

UPDATE myv5 SET Maximum Wage=10000 WHERE department_id=60;



#A subquery of the where clause references a table in the from clause

CREATE OR REPLACE VIEW myv6
AS

SELECT last_name,email,salary
FROM employees
WHERE employee_id IN(
	SELECT  manager_id
	FROM employees
	WHERE manager_id IS NOT NULL
);

#To update
SELECT * FROM myv6;
UPDATE myv6 SET salary=10000 WHERE last_name = 'k_ing';