MySQL Chapter 5 Summary

Create view
Basic grammar
have access to CREATE VIEW Statement to create the view .

The syntax is as follows ：

CREATE VIEW < View name > AS <SELECT sentence >

The grammar is as follows .
< View name >： Specifies the name of the view . The name must be unique in the database , Cannot have the same name as another table or view .
<SELECT sentence >： Specifies the time to create the view SELECT sentence , It can be used to query multiple base tables or source views .

For creating... In a view SELECT Statement has the following limitations ：

In addition to having CREATE VIEW Out of authority , It also has relevant permissions for the basic tables and other views involved in the operation . SELECT Statements cannot reference system or user variables .
SELECT Statement cannot contain FROM Subquery in Clause . SELECT Statement cannot reference preprocessing statement parameters .

The table or view referenced in the view definition must exist . however , After creating the view , You can delete tables or views that define references . You can use CHECK TABLE Statement to check the view definition for such problems .

View definitions allow the use of ORDER BY sentence , But if you choose from a specific view , This view uses its own ORDER BY sentence , Then... In the view definition ORDER BY Will be ignored .

Cannot reference... In view definition TEMPORARY surface （ A temporary table ）, Can't create TEMPORARY View .

WITH CHECK OPTION It means , When modifying the view , Check whether the inserted data conforms to WHERE Set the conditions .
Create a view based on a single table
MySQL You can create views on a single data table .

Be careful ：DESCRIBE In general, it can be abbreviated as DESC, Enter the execution result and input of this command DESCRIBE It's the same .
Modify the view
have access to ALTER VIEW Statement to modify the existing view .

The syntax is as follows ：

ALTER VIEW < View name > AS <SELECT sentence >

The grammar is as follows ：
< View name >： Specifies the name of the view . The name must be unique in the database , Cannot have the same name as another table or view .
<SELECT sentence >： Specifies the time to create the view SELECT sentence , It can be used to query multiple base tables or source views .

It should be noted that , about ALTER VIEW Use of statements , The user is required to have a view specific CREATE VIEW and DROP jurisdiction , And by
SELECT Some permissions on each column selected by the statement .

Modify the definition of the view , Except through ALTER VIEW Outside , You can also use DROP VIEW Statement to delete the view first , Reuse CREATE VIEW Statement to implement .

Update data table with view
The view is a virtual table , The actual data comes from the basic table , So by inserting 、 Modify and delete operations update the data in the view , It is essentially updating the data of the basic table referenced by the view .
Basic grammar

have access to ALTER VIEW Statement to modify the existing view .

The syntax is as follows ：

ALTER VIEW < View name > AS <SELECT sentence >

The grammar is as follows ：

< View name >： Specifies the name of the view . The name must be unique in the database , Cannot have the same name as another table or view .

<SELECT sentence >： Specifies the time to create the view SELECT sentence , It can be used to query multiple base tables or source views .

It should be noted that , about ALTER VIEW Use of statements , The user is required to have a view specific CREATE VIEW and DROP jurisdiction , And by SELECT Some permissions on each column selected by the statement .

Modify the definition of the view , Except through ALTER VIEW Outside , You can also use DROP VIEW Statement to delete the view first , Reuse CREATE VIEW Statement to implement .

Some views are updatable . in other words , have access to UPDATE、DELETE or INSERT Wait for the statement to update the content 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 base table .

Delete view
have access to DROP VIEW Statement to delete the view .

The syntax is as follows ：
DROP VIEW < View name 1> [ , < View name 2> …]

< View name > Specify the name of the view to delete .DROP VIEW Statement can delete multiple views at once , But you must have... On each view DROP jurisdiction .