Database view detailed exploration

One 、 Define views ：

1 View creation ：SQL Used in language CREATE VIEW Command to establish ;
Example ：

Create view <name> [< Name >[,< Name >]···)]
as < Subquery > with check option];

The subquery can be any select sentence

In relational database management system create view The result of the statement is to save the definition of the view into the data , They don't execute select sentence .

Two 、 Row column subset view

Derived from a single basic table , And just remove some rows and columns of the basic table , But the main code is retained , Then this kind of view is called row and column view .

create view IS_Student
as 
selcet Sno,Sname,Sage
from student
where Sdept='IS';

IS_Student The view can be regarded as a row column subset view ;

Views can not only be built on a single basic table , It can also be built on multiple basic tables ;

Views can not only be built on multiple basic tables , It can also be built on one or more defined views , Or based on basic tables and views .

3、 ... and 、 Delete view

sentence ：

drop view < View name >[cascade];

Four 、 Query view

Example : In the view of information department students, find out that they are younger than 20 Year old student .

select Sno,Sage
from IS_Student
where Sage<20;

5、 ... and Update the view

Update view is generally used insert（ Insert ）、delete（ Delete ）、update（ modify ） data ;
Example ：
View computer students IS_Student The middle school number is 2011050321 The student's name is changed to “ Zhang Yu ”.

update IS_Student
set Sname=' Zhang Yu '
where Sno='2011050321';

 The update statement after actual conversion is ：
update Student
set Sname=' Zhang Yu '
where Sno='2011050321' and Sdept='IS';

6、 ... and The function of view （ advantage ）

1 View can simplify the operation of users ;

2 Views enable users to view the same data from multiple perspectives ;

3 Views provide a certain degree of logical independence for refactoring the database ;

4 View can provide security for confidential data ;

5 Using views properly can express queries more clearly ;