SQL Server 2016 learning records - View

Learning goals ：

1、 Concept of view
2、 Define views
3、 Query view
4、 Update the view
5、 The function of view

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Learning content ：

1、 Concept of view
Views are virtual tables , From one or more basic tables （ Or view ） Exported table
Only store the definition of the view , Do not store the data corresponding to the view
The data in the base table changes , The data queried from the view also changes

2、 Define views

Create view
Sentence format
CREATE VIEW
< View name > [(< Name > [,< Name >]…)]
AS < Subquery >
[WITH CHECK OPTION];
Subqueries can be arbitrary SELECT sentence ;
The attribute column names that make up the view ： Omit all or specify all

-- Set up a view of information students .
CREATE VIEW IS_Student
AS 
    SELECT Sno,Sname,Sage
    FROM     Student
    WHERE  Sdept= 'IS';

-- Set up a view of information students , It is also required to ensure that the view is only for information department students when modifying and inserting operations  .
CREATE VIEW IS_Student
AS 
    SELECT Sno,Sname,Sage
    FROM  Student
    WHERE  Sdept= 'IS'
    WITH CHECK OPTION;

If a view is exported from a single base table , And just remove some rows and columns of the basic table , But the main code is retained , We call this kind of view row column subset view .

• View based on multiple base tables

-- Set up the Department of information 1 Student's view of Lesson No （ Including student ID 、 full name 、 achievement ）.
CREATE VIEW IS_S1(Sno,Sname,Grade)
AS 
SELECT Student.Sno,Sname,Grade
FROM  Student,SC
WHERE  Sdept= 'IS' AND
        Student.Sno=SC.Sno AND
        SC.Cno= '1';

• View based views

-- Set up the Department of information 1 And the grade is in 90 Views of students with a score above .
CREATE VIEW IS_S2
AS
SELECT Sno,Sname,Grade
FROM  IS_S1
WHERE  Grade>=90;

• Views with expressions

-- Define a view that reflects the year the student was born .
CREATE  VIEW BT_S(Sno,Sname,Sbirth)
AS 
SELECT Sno,Sname,2014-Sage
FROM  Student;

• Group view

-- Define the student number and grade average as a view 
CREATE  VIEW S_G(Sno,Gavg)
AS  
SELECT Sno,AVG(Grade)
FROM  SC
GROUP BY Sno;

Delete view
Format of statement ：
DROP VIEW < View name >[CASCADE];
This statement deletes the specified view definition from the data dictionary
If there are other views exported on this view , Use CASCADE Cascade delete statements , Delete this view along with all views exported from it
Error deleting base table , All view definitions exported from this base table must explicitly use DROP VIEW Statement delete

-- Delete view BT_S and IS_S1
DROP VIEW BT_S;	/* Successful execution */
DROP VIEW IS_S1;	/* Refuse to enforce */
/* To delete IS_S1, Cascade deletion is required ：DROP VIEW IS_S1 CASCADE; */    

3、 Query view
In fact, the query view is similar to the query table
The method of realizing view query in relational database management system ： View resolution method （View Resolution）

• Check the effectiveness
• Convert to an equivalent query to the basic table
• Execute the modified query

-- In the view of information students, find out that they are younger than 20 Year old student .
SELECT   Sno,Sage
FROM      IS_Student
WHERE   Sage<20;

4、 Update the view

-- View information department students IS_Student Middle school number ”201215122” The student's name was changed to ” Liu Chen ”.
UPDATE  IS_Student
SET  Sname= ' Liu Chen '
WHERE  Sno= ' 201215122 ';

-- To the information department students view IS_S Insert a new student record in , The student number is ”201215129”, The name is ” Zhao Xin ”, Age is 20 year 
INSERT
INTO IS_Student
VALUES('201215129',' Zhao Xin ',20);

Add data to the view , Is to add data to the master and apprentice's base table , Adding data also uses INSERT command , Be careful ：

• When trying to have multiple base tables , You cannot modify multiple base tables in a view at the same time in one statement , That is, you can only add, delete or modify data in one base table of the view at a time
• Because the view may only contain some columns of the base table , therefore , When inserting data into the base table through the view , It is required that other non view columns in the base table should be allowed to be empty or contain default values
• The inserted data must meet the constraints of the base table

5、 The function of view

• View can simplify the operation of users
• Views enable users to view the same data from multiple perspectives
• Views provide a certain degree of logical independence for refactoring databases
• View can provide security for confidential data
• The proper use of views can express queries more clearly