MySQL learning summary 10: detailed explanation of view use

1、 View overview

1.1 Why use views ？

   On the one hand, views can help us use some of the tables instead of all the tables , On the other hand, you can also make different query views for different users .
　　 such as , For a company's salesperson , We just want to show him some data , And some special data , For example, the purchase price , Will not be provided to him .
　　 Another example , Personnel compensation is a sensitive field , Then it is only open to people above a certain level , This field is not available in other people's query views .
　　 What I just talked about is just a usage scenario of the view , In fact, views have many functions .

1.2 View understanding

• View , It can be regarded as a virtual table , Itself does not store data . The essence of view , Can be seen as stored SELECT sentence .
• In the view SELECT The table involved in the statement , Called the base table .
• Do... For the view DML operation , It will affect the data in the corresponding base table . vice versa .
• Deletion of the view itself , It will not cause the deletion of data in the base table .
• Application scenarios of view ： For small projects （ For example, dozens of tables ）, Views are not recommended . For large projects （ For example, hundreds of tables ）, Consider using views .
• Benefits of views ： Simplify queries ; Control access to data .

2、 Create view

CREATE [OR REPLACE]
[ALGORITHM = {
   UNDEFINED | MERGE | TEMPTABLE}]
VIEW  View name  [( Field list )]
AS  Query statement 
[WITH [CASCADED|LOCAL] CHECK OPTION]

Lite version ：

CREATE VIEW  View name 
AS  Query statement 

2.1 Create a single table view

Take a chestnut ：

CREATE VIEW empvu80
AS
SELECT employee_id, last_name, salary
FROM   employees
WHERE   department_id = 80;

2.2 Create a multi table union view

Take a chestnut ：

CREATE VIEW emp_dept
AS
SELECT ename,dname
FROM t_employee LEFT JOIN t_department
ON t_employee.did = t_department.did;

2.3 Create a view based on the view

When we create a view , You can also continue to create views on top of it .

give an example : union “emp_dept” The view and “emp_year_salary” View query employee name 、 Department name 、 Annual salary information creation
“emp_dept_ysalary” View .

CREATE VIEW emp_dept_ysalary
AS
SELECT emp_dept.ename,dname,year_salary
FROM emp_dept INNER JOIN emp_year_salary
ON emp_dept.ename = emp_year_salary.ename;

3、 View view

grammar 1: View the table objects of the database 、 View objects

SHOW TABLES;

grammar 2: Look at the structure of the view

DESC / DESCRIBE  View name ;

grammar 3: View the attribute information of the view , Execution results show , notes Comment by VIEW, Explain that the table is a view , Other information is NULL, It means that this is a virtual table .

#  View view information ( Display the storage engine of the data table 、 edition 、 Number of data rows and data size, etc )
SHOW TABLE STATUS LIKE ' View name '\G

grammar 4: View the detailed definition information of the view

SHOW CREATE VIEW  View name ;

4、 Update the view

4.1 General situation

MySQL Support use INSERT、UPDATE and DELETE Statement to insert data in a view 、 Update and delete operations . When the data in the view changes , The data in the data table will also change , vice versa .

4.2 Non updatable views

To make the view updatable , Depending on the There must be... Between the rows in the diagram and the rows in the underlying basic table one-on-one The relationship between . In addition, when the view definition is as follows , View does not support update operation :

• When defining the view, you specify “ALGORITHM = TEMPTABLE”, View will not support INSERT and DELETE operation ;
• The view does not contain all columns in the base table that are defined as non empty and do not specify a default value , View will not support INSERT operation ;
• In defining the SELECT Used in the statement JOIN The joint query , View will not support INSERT and DELETE operation ;
• In defining the SELECT The field list after the statement uses Mathematical expression or Subquery , View will not support INSERT, Nor does it support UPDATE Mathematical expressions are used 、 The field value of the subquery ;
• In defining the SELECT Statement is used in the field list after the DISTINCT 、 Aggregate functions 、 GROUP BY 、 HAVING 、UNION etc. , View will not support INSERT、UPDATE、DELETE;
• In defining the SELECT Statement contains a subquery , The subquery refers to FROM The watch at the back , View will not support
  INSERT、UPDATE、DELETE;
• The view definition is based on a Non updatable view ;
• Constant view .

Although view data can be updated , But on the whole , View as Virtual table , It is mainly used for Convenient query , It is not recommended to update the data of the view . Changes to view data , It is done by operating the data in the actual data table .

5、 modify 、 Delete view

5.1 Modify the view

The way 1: Use CREATE OR REPLACE VIEW Clause to modify the view

CREATE OR REPLACE VIEW empvu80
(id_number, name, sal, department_id)
AS
SELECT
employee_id, first_name || ' ' || last_name, salary, department_id
FROM employees
WHERE department_id = 80;

The way 2:ALTER VIEW

ALTER VIEW  View name 
AS
 Query statement 

5.2 Delete view

Deleting a view just deletes the definition of the view , The data of the base table will not be deleted .

The syntax for deleting a view is :

DROP VIEW IF EXISTS  View name ;
DROP VIEW IF EXISTS  View name 1, View name 2, View name 3,...;

Be careful ： View based a、b Created a new view c, If the view a Or view b Delete , Will cause the view c Your query failed . this
Sample view c Need to manually delete or modify , Otherwise, it will affect the use of .

6、 Advantages and disadvantages of views

6.1 Benefits of views

１） It's easy to operate
　　 Define frequently used query operations as views , It can make developers do not need to care about the structure of the data table corresponding to the view 、 The relationship between tables , There is no need to care about the business logic and query conditions between data tables , Simply manipulate the view , It greatly simplifies the operation of developers on the database .
２） Reduce data redundancy
　　 The view is different from the actual data table , It stores query statements . therefore , In use , We want to get the result set by defining the query statement of the view . The view itself does not store data , Do not occupy the resources of data storage , Reduced data redundancy .
３） Data security
　　MySQL The user's response to the data Access restrictions On the result set of some data , The result set of these data can be realized by using views . Users do not have to query or manipulate the data table directly . This can also be understood as the view has Isolation, . The view is equivalent to adding a layer of virtual tables between the user and the actual data table .
　　 meanwhile ,MySQL Users' access to data can be restricted to some views according to their permissions , Users do not need to query the data table , The information in the data table can be obtained directly through the view . This ensures the security of the data in the data table to a certain extent .
４） Adapt to flexible needs
　　 When the requirements of the business system change , If you need to change the structure of the data table , The workload is relatively large , You can use views to reduce the amount of change . This method is often used in practical work .
５） Ability to decompose complex query logic
　　 If there is complex query logic in the database , be The problem can be decomposed , Create multiple views to get data , Then combine the created multiple views , Complete complex query logic .

6.2 Disadvantages of deletion

   If we create a view based on the actual data table , that , If the structure of the actual data table changes , We need to maintain the relevant views in time . Especially nested views ( Is to create a view based on the view ), Maintenance will become more complex , Poor readability , It can easily become a potential hidden danger of the system . Because the view is created SQL Queries may rename fields , It may also contain complex logic , These will increase the cost of maintenance .
　　 In actual projects , If there are too many views , It will lead to the problem of database maintenance cost .
　　 therefore , When creating views , you It should be combined with the actual project needs , Comprehensively consider the advantages and disadvantages of view , In this way, the view can be used correctly , Make the whole system optimal .