MySQL View

Introduce

View view It's a virtual table , It's not real , Its essence is based on SQL Statement to get a dynamic data set , And name it , Users only need to use the view name to get the result set , And you can use it as a table .

The database only stores the definition of the view , There is no data stored in view , These data are still put in the original data table .

 When using views to query data , The database system will take the corresponding data from the original data table . therefore , The data in the view depends on the data in the source data table , Source data change , The data displayed in the view will also change .

effect

Simplify the code , Reusable queries can be closed into views for reuse , At the same time, it can make complex queries easy to understand and use

Security reasons , If there are a lot of data in a table , A lot of information doesn't want to be seen by everyone , You can use the view , Such as ： There is a table that shows only names and some insensitive fields open to others , ID card and salary are not displayed , It can be applied to different users , Set different views .

View creation

Grammar format ：

create [or replace] [algorithm = {undefind | mgerge | templtable } }]
view view_name [ {column_list} ]
as select_statement
[with [cascaded | local ] check option ]

Parameter description ：

• algorithm: optional , Algorithm for view selection
• view_name： Represents the name of the view to be created
• column_list： optional , Specifies the noun for each attribute in the view , By default, it is the same as select The properties of the query in the statement are the same
• select_statement： Represents a complete query statement , Import the query record into the view
• [width [cascaded | local ] check option ]： optional , It means that updating the view should be within the permission range of the view

The operation sample ：

--  Create view 1

create or replace view view1_emp 
as
select emp_name from emp;

--  Query all tables , Differentiate view 
select full tables;

Modify the view

Modifying a view means modifying the definition of a table that already exists in the database . When some fields in this table change , You can modify the view to keep it consistent with this table .MySQL Pass through CREATE REPLACE VIEW Statement and ALTER VIEW Statement to modify the view .

Format ：

alter view  View name  as select  sentence 

Example ：

--  Modify the view 
alter view view1_emp
AS
select emp_name, salary from emp;

Update the view

Some views are updatable . in other words , Can be in UPDATE、DELETE perhaps INTER And so on , In a more basic form （ Data source table ） The content of . For updatable views , Rows in the view and rows in the base table must have a one-to-one relationship , If the view contains any of the following structures , So it's not updatable .

• Aggregate functions （SUM(), MIN(), MAX(), AVG(), COUNT() etc. ）
• DISTINCT
• GROUP BY
• HAVING
• UNION perhaps UNION ALL
• JOIN
• FORM Non updatable view in Clause
• WHERE Subquery in Clause , quote FROM Table in clause
• Reference text values only

Although the data can be updated in the view , But there are a lot of restrictions , In general , It's best to use the view as a virtual table for querying data , Instead of updating data through views . because , When using views to update data , If the limitation of updating data is not fully considered , Data update may fail .

Other operations of view

Rename view

--  Rename view 
rename table view1_emp to myview1;

Delete view

--  Delete view  
drop view if exists myview1