Summary of MySQL foundation view

1. View overview

1.1 Common database objects

object	describe
surface (TABLE)	A table is a logical unit that stores data , Exist as rows and columns , Columns are fields , Line is the record
The data dictionary	The system tables , A table that holds database related information . The data of the system table is usually maintained by the database system , Programmers usually shouldn't modify , You can only view
constraint (CONSTRAINT)	Rules for performing data verification , Rules for ensuring data integrity
View (VIEW)	Logical display of data in one or more datasheets , Views do not store data
Indexes (INDEX)	be used for Improve query performance , It's equivalent to a book catalog
stored procedure (PROCEDURE)	Used to complete once Complete business processing , no return value , However, multiple values can be passed to the calling environment through outgoing parameters
Storage function (FUNCTION)	Used to complete once Specific calculations , Has a return value
trigger (TRIGGER)	Equivalent to one Event listener , When a specific event occurs in the database , Trigger is triggered , Complete the corresponding processing

1.2 Reasons for using views

【 explain 】

     On the one hand, views can help us Use part of the table Not all the tables , On the other hand, it can also aim at Different users To develop Different query views . such as , For a company's salesperson , We just want to show him Part of the data , And some Special data , For example, the purchase price , Will not be provided to him . Another example , Personnel compensation is a sensitive Field of , Then it is only open to people above a certain level , Others' query views are This field is not provided .

1.3 View understanding

【 a key 】

① View , Can be seen as a Virtual table , Itself is Don't store data Of . The essence of view , Can be seen as Store it Of select sentence
② The tables involved in the view are called Base watch
③ For views DML operation , Meeting influence To the data in the base table
④ Deleting the data in the view will not affect the data in the table
⑤ Application scenarios in the view ： in the light of Small projects Not recommended . For large projects , Consider using views
⑥ Benefits of views ： Simplify queries , Control data access

2. Create view

① stay CREATE VIEW Insert subquery in statement

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

Lite version （ After simplification ）

CREATE VIEW  View name  
AS 
 Query statement 

2.1 Create a single table view

Preparation before creating a view , Create databases and tables

CREATE DATABASE dbtest14;

use dbtest14;


CREATE TABLE emps
AS 
SELECT *
FROM atguigudb.employees;

CREATE TABLE depts
AS
SELECT *
FROM atguigudb.departments;

Create a single table view

CREATE VIEW vu_emp1
AS
SELECT employee_id,last_name,salary
FROM emps;

The field is aliased after the view name

CREATE VIEW vu_emp3(id,name,salary)
AS
SELECT employee_id, last_name lname,salary
FROM emps
WHERE salary > 8000;

explain
    ① In fact, we are SQL Query statements encapsulation View VIEW, This will be based on SQL The result set of the statement forms a virtual table .
    ② When creating a view , Not after view name Specify the field list , The field list in the view defaults to and SELECT The list of fields in the statement is consistent . If SELECT Statement to the field Aliased , Then the field names in the view Same as alias .

2.2 Create a multi table union view

give an example ：
① Multiple tables pass JOIN ON The way

CREATE VIEW vu_emp_dept
AS
SELECT e.employee_id, e.department_id, d.department_name
FROM emps e JOIN depts d
ON e.department_id = d.department_id;

② Use the view to format the data

     We often need to output content in a certain format , For example, we want to output the employee name and the corresponding department name , The corresponding format is emp_name(department_name), You can use the view to complete the operation of data formatting ：

CREATE VIEW vu_emp_dept2
AS
SELECT CONCAT(e.last_name,'(',d.department_name,')') emp_info
FROM emps e JOIN depts d
ON e.department_id = d.department_id;

③ Create a view based on the view

CREATE VIEW vu_emp4
AS
SELECT id,name
FROM vu_emp3;

3. Manage attempts

3.1 View view

The way 1：
View the table objects of the database 、 View objects

SHOW TABLES;

The way 2：
Look at the structure of the view

DESC / DESCRIBE  View name ;

The way 3：
View the attribute information of the view

SHOW TABLE STATUS LIKE ' View name '\G 

The way 4：
View the detailed definition information of the view

SHOW CREATE VIEW  View name ;

3.2 Update data for view

Update view Syntax ：

UPDATE   View name 
SET  Update the content 
WHERE  Conditions 

SELECT * FROM vu_emp1;

UPDATE  vu_emp1
SET salary = 20000
WHERE employee_id = 102;

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

① When defining the view, you specify “ALGORITHM = TEMPTABLE”, View will not support INSERT and DELETE operation ;
② The view does not contain Base watch All are defined as Not empty and not specified Default value column , View will not support INSERT operation ;
③ In defining the SELECT Used in the statement JOIN The joint query , View will not support INSERT and DELETE operation ;
④ 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 ;
⑤ 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;
⑥ In defining the SELECT The statement contains Subquery , The subquery refers to FROM The watch at the back , View will not support INSERT、UPDATE、DELETE;
⑦ The view definition is based on a Non updatable view ;
⑧ Constant View .

3.3 modify 、 Delete view

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

CREATE OR REPLACE VIEW vu_emp1
AS
SELECT employee_id,last_name,salary,email
FROM emps;

The way 2：ALTER VIEW

ALTER VIEW vu_emp1
AS
SELECT employee_id,last_name,salary,email,hire_date
FROM emps;

Delete view

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

Use the syntax ：

DROP VIEW IF EXISTS  View name ;

explain ：
     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 . Such a view c Need to manually delete or modify , Otherwise, it will affect the use of .

3.4 View summary

1. It's easy to operate

     Define frequently used query operations as View , It can make developers do not need to care about the data table corresponding to the view structure 、 Between tables Connections , There is no need to care about the relationship between data tables Business logic and Query criteria , And simply Operation view that will do , It greatly simplifies developers' understanding of Operation of database .

2. Reduce data redundancy

     View with Actual data sheet Dissimilarity , It stores Query statement . therefore , In use , We will define the query statement of the view to Get the result set . And the view itself Don't store data , Do not occupy the resources of data storage , Reduce data redundancy .

3. Data security

    MySQL The user's response to the data Access restrictions On the result set of some data , And these data Result set You can use views to implement . Users do not have to Direct inquiry or Operation data sheet . This can also be understood as the view has Isolation, . The view is equivalent to adding a layer between the user and the actual data table Virtual 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 . To a certain extent, this ensures that the data in the data table Security .

4. Adapt to flexible needs

     When the requirements of the business system change , if necessary Change data sheet Structure , The workload is relatively large , You can use views to Reduce changes The amount of work . This method is often used in practical work .

5. Ability to decompose complex query logic

     If there are complex Query logic , Then the problem can be decompose , Create multiple views get data , Then combine the created multiple views , Complete complex query logic . If we create a view based on the actual data table , that , If the actual data sheet Structural change , We need to update the relevant views in time Corresponding maintenance . 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 .