I learned database at station B (10): View

Last one ： I am here b Station learning database （ Nine ）： function

One 、 Introduce
1、 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 , When using, users only need to use the view name to obtain the result set , And you can use it as a table .

2、 In the database Only the definition of the view is stored , There is no data stored in view . The data is stored in the original table .

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

Two 、 effect
Security reasons , If there are a lot of data in a table , A lot of information doesn't want everyone to see , You can use the view view , Such as ： Social security fund table , You can use the view to display only names , Address , Instead of showing the social security number and the number of wages , It can be applied to different users , Set different views .

3、 ... and 、 Create view
1、 grammar ：
create [or replace] [algorithm = {undefined | merge | temptable}]

view view_name [(column_list)]

as select_statement

[with [cascaded | local] check option]

Parameter description ：
（1）algorithm： optional , Algorithm for view selection .
（2）view_name ： Represents the name of the view to be created .
（3）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 .
（4）select_statement
： Represents a complete query statement , Import the query record into the view .
（5）[with [cascaded | local] check option]： optional , It means that when updating a view, it must be within the permission range of the view .

2、 Data preparation
Create three tables in one database
dept surface （ department ）emp surface （ staff ）
salgrade surface （ Pay scales ）

3、 example

create or replace view view1_emp
as 
select ename,job from emp; 

--  View tables and views  
show full tables;

Four 、 Modify the view
1、MySQL Pass through CREATE OR REPLACE VIEW Statement and ALTER VIEW Statement to modify the view .
2、 Format
alter view View name as select sentence
3、 example

alter view view1_emp
as 
select a.deptno,a.dname,a.loc,b.ename,b.sal from dept a, emp b where a.deptno = b.deptno;

5、 ... and 、 Update the view
1、 If the view contains any of the following structures , So it's not updatable ：
Aggregate functions （SUM(), MIN(), MAX(), COUNT() etc. ）
DISTINCT
GROUP BY
HAVING
UNION or UNION ALL
Subqueries in the selection list
JOIN
FROM Non updatable view in Clause
WHERE Subquery in Clause , quote FROM Table in clause .
Reference text values only （ In this case , There are no basic tables to update ）

Be careful ： 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 you don't fully consider the limitations of updating data in the view , Data update may fail .

2、 example

--  --------- Update the view -------
create or replace view view1_emp
as 
select ename,job from emp;
 
update view1_emp set ename = ' Zhou Yu ' where ename = ' Lu Su ';  --  You can modify 
insert into view1_emp values(' king of Wu in the Three Kingdoms Era ',' Clerk ');  --  You can't insert 
# Be careful ： Update and insert data , It's all true emp Table operation 

-- ---------- The view contains aggregate functions that are not updatable --------------
create or replace view view2_emp
as 
select count(*) cnt from emp;
 
insert into view2_emp values(100);   # Report errors 
update view2_emp set cnt = 100;    # Report errors 

-- ---------- The view contains distinct Not updatable ---------
create or replace view view3_emp
as 
select distinct job from emp;
 
insert into view3_emp values(' financial ');

-- ---------- The view contains goup by 、having Not updatable ------------------
 
create or replace view view4_emp
as 
select deptno ,count(*) cnt from emp group by deptno having  cnt > 2;
 
insert into view4_emp values(30,100);

-- ---------------- The view contains union perhaps union all Not updatable ----------------
create or replace view view5_emp
as 
select empno,ename from emp where empno <= 1005
union 
select empno,ename from emp where empno > 1005;
 
insert into view5_emp values(1015,' Trinket ');

-- ------------------- The view contains subqueries that are not updatable --------------------
create or replace view view6_emp
as 
select empno,ename,sal from emp where sal = (select max(sal) from emp);
 
insert into view6_emp values(1015,' Trinket ',30000);

-- ---------------------- The view contains join Not updatable -----------------
create or replace view view7_emp
as 
select dname,ename,sal from emp a join  dept b  on a.deptno = b.deptno;
 
insert into view7_emp(dname,ename,sal) values(' The administration department ',' Trinket ',30000);
 
-- -------------------- The view contains constant text values that are not updatable -----------------
create or replace view view8_emp
as 
select ' The administration department ' dname,' Yang2 guo4 '  ename;
 
insert into view8_emp values(' The administration department ',' Trinket ');

6、 ... and 、 Rename view
1、 Format
rename table View name to New view name ;
2、 example

rename table view1_emp to my_view1

7、 ... and 、 Delete view
1、 Format
drop view View name [, View name …];
2、 example

drop view if exists view_student;

Be careful ： When deleting a view , You can only delete the definition of a view , Data will not be deleted .