MySQL view concept, create view, view, modify view, delete view

1 Concept of view

• View is a virtual table , There is no data in itself , Takes up very little memory space , It is SQL An important concept in .
• Views are built on existing tables , The tables on which the view is built are called Base watch .
  
• The creation and deletion of views only affect the view itself , It does not affect the corresponding base table . But when the data in the view is added 、 When deleting and modifying operations , The data in the data table will change accordingly , vice versa .
• The statement that provides data content to the view is SELECT sentence , A view can be understood as Stored up SELECT sentence .
• In the database , The view does not save data , The data is really saved in the data table . When adding data to the view 、 When deleting and modifying operations , The data in the data table will change accordingly ; vice versa .
• View , Is another form of providing users with base table data . Usually , Databases for small projects may not use views , But on big projects , And when the data table is complex , The value of view is highlighted , It can help us put the result set of frequent queries into the virtual table , Improve efficiency . It is very convenient to understand and use .

2 Create view

2.1 grammar

• 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

CREATE VIEW  View name  [( Field list )]
AS  Query statement 

2.2 Name the field

--  Mode one 
CREATE VIEW emp_view AS SELECT
employee_id emp_id,--  The column name of the view is consistent with the alias 
last_name lname,
salary 
FROM
	employees;


--  Mode two 
--  The name should correspond to the query field one by one 
CREATE VIEW emp_view2 ( emp_id, lname, money ) AS SELECT
employee_id,
last_name,
salary 
FROM
	employees;

3 View view

--  View tables and views  
SHOW TABLES;
--  View view structure 
DESC emp_view2;
--  View the attribute information of the view 
SHOW TABLE STATUS LIKE 'emp_view2';
--  View the detailed definition information of the view 
SHOW CREATE VIEW emp_view2;

4 Modify the view

--  Create a view to modify 
CREATE VIEW emp_view AS SELECT
employee_id emp_id,
last_name lname,
salary 
FROM
	employees;

--  Modification method 1 
CREATE OR REPLACE VIEW emp_view AS SELECT
employee_id,
last_name,
salary,
email 
FROM
	employees 
WHERE
	salary > 7000;
	
--  Modification method 2 
ALTER VIEW emp_view AS SELECT
employee_id emp_id,
last_name lname,
salary 
FROM
	employees;

5 Delete view

--  Delete view 
	DROP VIEW IF EXISTS emp_view;
	DROP VIEW IF EXISTS emp_view,emp_view2;