MySQL | view notes on Master Kong MySQL from introduction to advanced

1. View concept

• Create a view from a table , Then this table is called Base watch .

• A view is a Virtual table , amount to SELECT Encapsulation of statements .

  Cover up Sheepskin SELECT Of The Wolf View

• Create view perhaps Delete view In itself , Does not affect the base table .
  

2. Why use views ？—— Query query ！

For convenience Inquire about . Repeat three times , remember , Only for query ！
When you just want to show specific content to specific people , Using views can simplify queries .

Image memory ： The view is like a magic mirror , It only shows a part of the base table that allows you to see .

（1） advantage

Compared with the new word table , It can reduce the redundancy of data .
Views are isolated , Protect raw data , Help data security .
When the query is complex , You can create multiple views , Then integrate the results of these views , Complete complex queries .

therefore , For view , Don't get rid of those fancy things , Such as adding, deleting and modifying ！
The view is a virtual window , So do it DML（ Additions and deletions ） In operation , Directly to Base watch Conduct DML operation .

• insert data INSERT（ When the fields in the base table have constraints , Insert data if it does not conform to the constraint , The insertion fails ）
• Update data UPDATE
• Delete data DELETE
  Fields in view , There may be no corresponding field in the base table . such as , Fields calculated from the base table . For these fields , It's impossible to execute UPDATE、DELETE Operation of the .
  Just look at this passage , It is not recommended to use ！

（2） shortcoming

Maintenance costs are high

3. Create view

CREATE OR REPLACE VIEW View Can be used directly Create or modify views . In fact, it is basically written like this .
.
Because it is not mentioned here Modify the view , So first, simply use CREATE VIEW To introduce .

（1） Single table

CREATE VIEW  View name ( Field alias 1,  Field alias 2, ...)  --  If it has the same name as the field in the query , Alias names can be omitted `( Field alias 1,  Field alias 2, ...)`
AS
SELECT  Field 1,  Field 2, ...
FROM  Table name 
WHERE  Filter conditions 

（2） Multiple tables

CREATE VIEW  View name ( Field alias 1,  Field alias 2, ...)  --  If it has the same name as the field in the query , Alias names can be omitted `( Field alias 1,  Field alias 2, ...)`
AS
SELECT  Table name . Field 1,  Table name . Field 2, ...
FROM  Table name 1,  Table name 2
ON  Table name 1. Field  =  Table name 2. Field ;
WHERE  Filter conditions 

（3） Create a view based on the view

hold View regard as surface Just write the code , There is no difference in the code .

4. View view

（1） Show the view

SHOW TABLES;

（2） Look at the structure of the view

DESC View name

（3） See the details of the view

SHOW CREATE VIEW View name

（4） Query all contents in the view

SELECT *
FROM  View name 

5. Modify the view

Common topics ： take ... Restriction on ... Within the scope of

（1） Add one more OR REPLACE Just go

You can use this code directly Create or modify views .

CREATE OR REPLACE VIEW  View name  
AS
 Query statements for base tables 

（2）ALTER VIEW

ALTER TABLE  View name 
AS 
 Query statements for base tables 

6. Delete view

DROP VIEW IF EXISTS View name