[mysql] database - View

What is a view

• The view displays data from one or more tables in a customized manner
• A view is a database object （ have access to create Keyword create view ）, The user can query the view like a normal table
• No data is actually stored in the view , It's just a query to the table （ What is stored in the view is SQL sentence , Not the result . The reason is because , When the previous SQL When the statement is modified , If the view is stored, execute the results （ The result will not change ）, Then the result will be after the change SQL Statements will conflict ）
• The definition of the view is saved in the data dictionary , The table on which the view is created is called “ Base watch ”

Why you need a view

If you often need to perform join queries on two tables , You have to connect the tables every time , Write the same string of statements , At the same time, some information may be sensitive and do not want to be seen by ordinary users . In this case, we can solve the problem through the view .

The role of view is somewhat

effect ：

• Control security
• Save query data

somewhat ：

• Provides flexible and consistent levels of security
• Hide the complexity of the data
• Simplify the user's SQL Instructions
• By means of rebranding , Provide data from another perspective

Environmental preparation ：
 

#  Create department table 
CREATE TABLE dept(
   did INT PRIMARY KEY AUTO_INCREMENT,
   dname VARCHAR(20)
  );
#  Create an employee table 
CREATE TABLE emp (
   id INT PRIMARY KEY AUTO_INCREMENT,
   NAME VARCHAR(10),
   gender CHAR(1), --  Gender 
   salary DOUBLE, --  Wages 
   join_date DATE, --  Date of entry 
   dep_id INT,
    FOREIGN KEY (dep_id) REFERENCES dept(did) --  Foreign keys , Related department table ( The primary key of the Department table )
);
--  Add Department data 
INSERT INTO dept (dNAME) VALUES (' R & D department '),(' The Marketing Department '),(' Finance Department '),(' The sales department ');
--  Add employee data 
INSERT INTO emp(NAME,gender,salary,join_date,dep_id) VALUES
(' The Monkey King ',' male ',7200,'2013-02-24',1),
(' Pig eight quit ',' male ',3600,'2010-12-02',2),
(' Tang's monk ',' male ',9000,'2008-08-08',2),
(' Bones jing ',' Woman ',5000,'2015-10-07',3),
(' Spider essence ',' Woman ',4500,'2011-03-14',1),
(' Small white dragon ',' male ',2500,'2011-02-14',null);

Create view

  The essence of view is SQL Instructions （select sentence ）, Basic grammar ：

CREATE VIEW  View name  AS SQL sentence ;

  When we just want to show the employee's name and department to ordinary users , When other information is hidden, we can write this ：

select 
emp.name  Employee's sex name , dept.d_name  Department 
from
emp,dept 
where
emp.dep_id = dept.d_id;

Execution results ： 

  Analyze the view above , I believe you have a certain understanding of the view . A view is a virtual table , When we want to turn the query results of the above two tables into one table , Then I can use the view , The statement is as follows

create view v_emp_dept 
as 
(select emp.name  Employee's sex name , dept.d_name  Department  from emp,dept where emp.dep_id = dept.d_id);

Use view

         The view is a virtual table , You can directly think of the view as ” surface ” operation , But the view itself has no data , Is temporary select Statement to get the corresponding result , Views are mainly used for query operations . Basic grammar

select  Field list  from  View name  [ Clause ];

  Inquire about v_emp_dept View

SELECT * FROM v_emp_dept;            --  Single table query 

The execution result is ：

 

Rules for using views

1. Views must have unique names
2. stay MySQL There is no limit to the number of views in
3. To create a view, you must obtain the necessary permissions from the administrator
4. Views support nesting , That is to say, a new view can be created by using the data retrieved from other views
5. It can be used in the view OREDR BY, However, if the sort clause is already used in the view , Then the view's ORDER BY Will overwrite the previous ORDER BY
6. View cannot be indexed , You cannot associate triggers or default values （ A view is not essentially a table ）
7. Views can be used with tables

Modify the view

Use CREATE OR REPLACE VIEW Statement modification v_emp_dept View . Specify a column name for each column .
 

create
or replace 
view v_emp_dept 
as 
(select emp.id, emp.name  Employee's sex name , dept.d_name  Department  from emp,dept where emp.dep_id = dept.d_id);

--  Query view 

SELECT * FROM v_emp_dept;

Execution results ：

  stay CREATE VIEW The fields in the statement must correspond to the fields in the subquery one by one , Otherwise, don't specify an alias , Or specify an alias to use in the subquery ALTER VIEW Statement modification EMP_V_10 View . Specify a column name for each column .

alter view v_emp_dept 
( Number ,  Wages ,  full name ,  department ) 
as (select emp.id, emp.salary, emp.name , dept.d_name 
from emp,dept 
where 
emp.dep_id = dept.d_id);

--  Query view 
SELECT * FROM v_emp_dept;

Execution results ：

 

Be careful ： It is not recommended to add, delete or modify the view data . In case it has to be revised , What essentially changes is the structure of the table

Delete view

Deleting views does not result in data loss , Because a view is a query definition based on a database table .

DROP VIEW  View name ;