Mysql database foundation: views and variables

View

1、 meaning

Simply understand that a view is a virtual table , The data for rows and columns comes from the tables used in queries that define views , And it's dynamically generated when using views , Only saved sql Logic , Do not save query results .

• Use scenarios
  The same query results are used in multiple places , The query result uses sql The statement is more complex .

2、 View features

1、 favour and put sb. in important position sql sentence
2、 Simplify complex sql operation , You don't have to know its query details
3、 Protection data , Improve safety （ Only provide data that customers want to know ）

3、 establish / Modify the view

• grammar

#  Mode one ：
create view  View name 
as
 Query statement ;

#  The following two statements can also be used to modify the view 
#  Mode two 
create or replace view  View name 
as
 Query view ;

#  Mode three 
alter view  View name 
as
 Query view ;

4、 View view

#  Mode one 
desc  View name ;

#  Mode two 
show create view  View name ;

5、 Delete view

drop view  View name 1, View name 2...;

6、 Update the view

Update the data in the view

• grammar

#  Insert 
insert into  View name  varlues(' value 1',' value 2',...);

#  modify 
update  View name  set  Name =' value ',... where  Conditions 
;

#  Delete 
delete from  View name  where  Conditions ;

It should be noted that the view update operation is generally not allowed , Because the update of the view will cause the data in the original table to change
The following types of views cannot be updated

7、 Comparison between view and table

type	Create keywords for Syntax	Whether it actually takes up physical space
View	create	Only saved sql Logic , It doesn't take up space
surface	create table	Saved data , Occupancy space

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Variable

1、 classification

• System variables
  • Global variables
  • Session variables
• Custom variable
  • User variables
  • local variable

2、 System variables

• explain
  Variables are provided by the system , Not user defined , At the server level .

• Scope

  • Global variables ： Every time the server starts, it will assign initial values to all global variables , Valid for all session connections .
  • Session variables ： Only valid for the current session connection .

• grammar
  1、 Look at all the system variables
  show global|【session】 variables

  2、 Look at some of the system variables that meet the criteria
  show global|【session】variables like '%char%';

  3、 View the value of a specified system variable
  select @@global|【session】. System variable name

  4、 Assign a value to a system variable
  Mode one ：
  set global|【session】 System variable name = value ;
  Mode two ：
  set @@global|【session】. System variable name = value ;

Add ： If it is global level , You need to add global, If it's session level , You need to add session, If you do not write, the default is session

3、 Custom variable

• explain ：
  Variables allow users to define themselves , Not defined by the system
• Use steps ：
  (1) Statement (2) assignment (3) Use （ see 、 Compare 、 Operations, etc ）

3.1 User variables

• Scope ： The pin is valid for the current session connection , Can be applied anywhere , That is to say begin end Inside or begin end outside

1. Declare and initialize

Mode one ：set @ User variable name = value ;
Mode two ：set @ User variable name := value ;
Mode three ：select @ User variable name := value ;

2. assignment （ Update the value of the user variable ）

Mode one ： adopt set or select

Mode one ： set @ User variable name = value ;
Mode two ： set @ User variable name := value ;
Mode three ： select @ User variable name := value ;

Mode two ： adopt select into

select  Field  into  Variable name  from  surface ;

#  for example ： take count(*) The obtained value is assigned to @count Go to variables 
select count(*) into @count;

3. Use （ Look at the value of the user variable ）

select @ User variable name ;

3.2 local variable

Scope ： Just defining it begin end Effective in , Apply to begin end The first sentence of

1. Statement

Mode one ：declare Variable name type ;
Mode two ：declare Variable name type default value ;

2. assignment

Mode one ：

Mode one ： set Local variable name = value ;
Mode two ： set Local variable name := value ;
Mode three ： select Local variable name := value ;

Mode two ： adopt select into

select  Field  into  Local variable name  from  surface ;

3. Use

select  Local variable name ;