[MySQL] introduction, function, creation, view, deletion and modification of database view (with exercises)

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This article is from the column ：MySQL8.0 Learning notes
This article refers to the video ：MySQL Database complete tutorial
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1 Introduction and function of view

Introduction to view ：

• View view It's a virtual table , It's not real , Its The essence is the basis 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 .
• Only the definition of view is stored in the database , There is no data stored in view . The data still exists in the original data table .
• 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 . When the data of the table changes , The data in the view will also change .

The function of view ：

• Simplify the code , We can encapsulate reused queries into views for reuse , Also can Make complex queries easy to understand ;
• More secure , such as , If there is a table with a lot of data , A lot of information doesn't want to be seen by others , Then you can use the view , Use different views for different users .

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2 View creation

The syntax for creating a view is as follows ：

create [or replace] [algorithm = {
   undefined | merge | temptable}]
view view_name [(column_list)]
as select_statement
[with [cascaded | local] check option]

Parameter description ：

1. algorithm： Algorithm for view selection , Optional ;
2. view_name： Name of the created view ;
3. column_list： 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]： It means to ensure that the view is within the permission range when updating the view .

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3 View modification

Modifying a view means modifying the definition of a table that already exists in the database . When some fields in the basic table change , You can modify the view to keep the consistency between the view and the basic table .

️ Grammar format ：

alter view  View name  as select sentence ;

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4 Update of view

Not all views can be updated . Can be in UPDATE、DELETE or INSERT And so on , To update the contents of the basic table . For updatable views , There must be a one-to-one relationship between the rows in the view and the rows in the basic table , If the view contains any of the following structures , The view is not updatable ：

1. Aggregate functions （SUM()、MIN()、MAX() etc. ）;
2. DISTINCT;
3. HAVING;
4. UNION perhaps UNION ALL;
5. Subqueries in the selection list ;
6. JOIN;
7. FROM Non updatable view in Clause ;
8. WHERE Subquery in Clause , quote FROM Table in clause ;
9. Use only literal values （ In this case , There are no basic tables to update ）.

Be careful ：
Although the data can be updated in the view , But there are many limitations . In general , It's best to use the view as a virtual table for querying data , Instead of updating data through views .
When a field existing in the view is modified in the real table , The view needs to be updated , Otherwise, the view will become invalid ！

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5 Rename and delete views

Rename view ：

rename table  View name  to  New view name ;

Delete view ：

drop view if exists  View name ;

When deleting a view , Only the definition of the view is deleted , It will not delete the data in the real table

If you want to delete multiple views at the same time , Then use the following syntax format ：

drop view if exists  View name 1,  View name 2,  View name 3...;

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6 View exercises

6.1 Data preparation

When practicing, you can first create two basic tables for practice according to the following code ：

create table college
(
    cno   int         null,
    cname varchar(20) null
);

create table student
(
    sid     int         null,
    name    varchar(20) null,
    gender  varchar(20) null,
    age     int         null,
    birth   date        null,
    address varchar(20) null,
    score   double      null
);

The basic data of the two tables are shown in the figure below ：

6.2 Query the name of the school with the highest average score

Combined with the knowledge learned before, you can Try subqueries and join queries To achieve , The reference codes are as follows ：

SELECT cname
FROM (SELECT cname, rank() over (order by avg_score desc ) item
      FROM (SELECT cname, avg(score) avg_score
            FROM student
                     JOIN college ON sid = cno
            GROUP BY cname) t) tt
WHERE item = 1;

In the above code , First the student And college The two tables are related , Treat the associated query as a sub table , And sort the average according to the sub table , The average number is 1 Has the highest average score , Then take this as a sub table for sub query , The school with the highest average score was found . The results are as follows ：

Although this method can solve the problem , But it's relatively complicated , It's not easy to understand , To simplify the code , We can create each subquery as a view

View solution code ：

-- 1  View one , Connect the two tables and calculate the average 
CREATE VIEW t_view AS
SELECT cname, avg(score) avg_score
FROM student
         JOIN college ON sid = cno
GROUP BY cname;

-- 2  View II , Use view 1 to sort and label the average scores 
CREATE VIEW tt_view AS
SELECT cname, rank() over (order by avg_score desc ) item
FROM (t_view);

-- 3  Use view to query 
SELECT cname
FROM (tt_view)
WHERE item = 1;

After creating the view , If you want to check the top three schools with average scores , It's a lot more convenient , The created view can be used directly ！

Reference code and results ：

SELECT cname
FROM (tt_view)
WHERE item = 1
   OR item = 2
   OR item = 3;

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At the end

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