Mysql database learning (8) -- MySQL content supplement

View

 What is a view ？
	 View is to get a virtual table through query , Then save , You can use it directly next time 
    
 Why use views ？
	 If you want to operate a virtual table frequently （ Make up a table ）, It can be made into a view , And then continue to operate 
    
#  grammar 
create view  View name  as  Query of virtual table sql sentence 
#  give an example 
create view emp2dep as select * from emp inner join dep on emp.dep = dep.id;

 Be careful 
	 When the view is saved, there will only be table results , No data （ The data still comes from the previous table ）
	 Views are generally only used to query , The data inside does not need to be modified , May affect the real table 
	
 View usage frequency 
	 Not much ,
	 When you create many views , It will make it difficult to maintain the table 

trigger

'''  Increase the table data when it meets the requirements 、 Delete 、 The function of automatically triggering in case of change   Using triggers can help us monitor 、 journal ...  Trigger can be in  6  Automatically triggered in case of   increase 、 Delete 、 Before and after the change  '''

 Basic grammar 
	create trigger  Trigger name  before/after insert/update/delete on  Table name  for each row begin sql sentence  end;
	
#  Specific use   The name of the trigger   We need to   See the name and know the meaning 
create trigger tri_before_insert_ti before insert on t1 for each row begin sql sentence  end;

 modify  MySQL  Default statement Terminator , Works only on the current window 
	delimiter $$  #  Change the default ending symbol to  $$
	delimiter ;
	
#  Case study ( Similar to the error log function )
create table cmd(
	id int primary key auto_increment,
    user char(32),
    priv char(10),
    cmd char(64),
    sub_time datetime,
    success enum("yes", "no") # 0 Delegate execution failed 
);

create table errlog(
	id int primary key auto_increment,
    err_cmd char(64),
    err_time datetime
);

'''  When  cmd  In the table  success  The fields are  no  Then trigger execution  errlog  Inject data into the table  NEW  It refers to data objects  '''

delimiter $$

create trigger tri_insert_after_cmd after insert on cmd for each row
begin 
	if NEW.success = "no" then
		insert into errlog(err_cmd, err_time) values (NEW.cmd, NEW.sub_time);
	end if;
end $$

delimiter ;

#  To  cmd  Table insert data 
insert into cmd (
	user,
    priv,
    cmd,
    sub_time,
    success
)
values
	("aoteman", "0755", "ls -1 /etc", NOW(), "yes"),
	("aoteman", "0755", "cat /etc/password", NOW(), "no"),
	("aoteman", "0755", "useradd xxx", NOW(), "no"),
	("aoteman", "0755", "ps aux", NOW(), "yes");
	
#  Delete trigger 
drop trigger tri_insert_after_cmd;

Business

 What is business 
	 Starting a transaction can contain multiple SQL sentence , these  sql  The statement either succeeds at the same time , Either don't think about success , Call it the atomicity of transactions 
	
 The role of affairs 
	 Ensure the security of data operation 
	
 Four characteristics of transactions （ACID）
A： Atomicity 
	 A transaction is an indivisible unit , Many operations contained in a transaction , Or at the same time , Or fail at the same time 
	
C:  Uniformity 
	 The transaction must be to change the database from one consistent state to another consistent state 
	
I：  Isolation, 
	 The execution of one transaction cannot be interfered by other transactions ,
	 That is, the operation and data used within a thing are isolated from other concurrent transactions , Concurrent transactions do not interfere with each other 	
	
D：  persistence 
	 It's also called permanence 
	 Once a transaction is executed successfully , Changes to database data should be permanent , The following other operations or faults should not have any impact on the alignment 
	
 How to use transactions 
1.  keyword 
start transaction;
2.  Rollback operation , Return to the state before the transaction is executed 
rollback;
3.  Secondary confirmation , Cannot rollback after confirmation 
commit;

" Analog transfer "
create table user(
	id int primary key auto_increment,
    name char(16),
    balance int
);

insert into user(name, balance) values
("a", 1000),
("b", 1000),
("c", 1000);

#  Start transaction first 
start transaction;
#  multiple  sql  sentence 
update user set balance=900 where name = "a";
update user set balance=1010 where name = "b";
update user set balance=1090 where name = "c";

rollback;

commit;

stored procedure

 Be similar to  python  Medium   Custom function 
  It contains a series of executable  sql  sentence , Stored procedures are stored in  Mysql  Server side , Trigger can be used directly sql Statement execution 
 
 delimiter $$
 
 create procedure  Name of stored procedure ( Shape parameter 1,  Shape parameter 2)
 begin
 	sql  sentence 
 end
 
 delimiter ;
 
 call  Name of stored procedure ();
 
  Three development models 
 	1.  Application programmers write code for development ,MySQL  Write stored procedures in advance , Supply the program call ——（ advantage ） Improve development efficiency , Improve the efficiency of execution ;（ shortcoming ） Considering the human element 、 Cross departmental communication issues , The scalability of subsequent stored procedures is poor 
 	
 	2.  Applications ： Programmers write code outside of development , The database operation is also written by myself ——（ advantage ） High expansibility ;（ shortcoming ） Reduced development efficiency , To write sql The sentence is too complicated , You need to think about sql The problem of optimization 
 	
 	3.  Applications only write code , Don't write sql sentence , Based on what others have written MySQL Of python frame （ORM frame ） It can be operated directly ——（ advantage ） High development efficiency ;（ shortcoming ） Because it is encapsulated by others , Poor extensibility of statements , There may be inefficiencies 

Stored procedure demonstration

delimiter $$

create procedure d1(
	in m int, #  Can't get in , m  Can't go back out 
	in n int,
    out res int #  This parameter can be returned 
)
begin
	select * from emp where id > m and age > n;
	set res=0; #  take  res  Variable modification , Used to identify that the current stored procedure code is actually executed 
end$$

delimiter ;

call d1(1, 28, 10); #  Report errors 

#  in the light of   Shape parameter  res  Can't transfer data directly   Should pass a variable name 
#  Defining variables 
set @ret = 10;
#  Look at the value of the variable 
select @ret;

call d1(1, 28, @ret)

stay pyMySQL How to call

import pymysql
......
......
#  Calling stored procedure 
cursor.callproc(d1, (1, 5, 10))
print(cursor.fetchall())

function

 It's different from stored procedures , It is equivalent to that the stored procedure is a user-defined function , Functions are built-in functions 

 for instance ： NOW()

create table blog(
	id int primary key auto_increment,
    name char(32),
    sub_time datetime
);

insert into blog(name, sub_time)
values
	(" The first 1 piece ", "2015-03-01 11:31:21"),
	(" The first 2 piece ", "2015-03-11 16:31:21"),
	(" The first 3 piece ", "2016-07-01 10:21:21"),
	(" The first 4 piece ", "2016-07-22 09:23:21"),
	(" The first 5 piece ", "2016-07-23 10:11:11"),
	(" The first 6 piece ", "2016-07-25 11:21:31"),
	(" The first 7 piece ", "2017-03-01 15:33:21"),
	(" The first 8 piece ", "2017-03-01 17:32:21"),
	(" The first 9 piece ", "2017-03-01 18:31:21");
	
select date_format(sub_time, "%Y-%m"), count(id) from blog group by date_format(sub_time, "%Y-%m");

Process control

# if  Judge 
delimiter //
create procedure proc_if()
begin
	declare i int defalut 0;
	if i=1 then
		select 1;
	elseif i =2 then
		select 2;
	else
		select 7;
	end if;
end //

delimiter ;

# while  loop 
delimiter //
create procedure proc_while()
begin
	declare num int ;
	set num = 0;
	while num < 10 do
		select 
			num;
		set num = num - 1;
	end while;
end // 

delimiter ;

Indexes

 The data is stored on the hard disk , Querying data inevitably requires  IO operation 

 Indexes ： It's a data structure , A catalogue similar to a book . It means that when querying data, you first find the directory and then the data , So as to improve the query speed and reduce IO operation 

 Index in MySQL It's also called “ key ”, It is a data structure for storage engine to quickly find records （innodb,）

primary key
unique key
index key

 Be careful  foreign key  It is not used to speed up the query 

 The three above  key, In addition to increasing the query speed, the first two have constraints , and  index key  There are no constraints , Just to help you quickly query data 

 The essence 
	 Constantly narrow the desired data range and filter out the final results , At the same time, random events （ Turn page by page ） Become sequential events （ Find the directory first 、 To find the data ）
	 That is to say, with index data , We can always find data in a fixed way 
	
 There can be multiple indexes in a table （ Multiple directories ）, For example, Xinhua dictionary can be looked up with radicals , You can also use pinyin to check .

 Although indexing can help you speed up your query , There are also shortcomings 

1.  When there is a large amount of data in the table , Index creation will be slow 
2.  After the index is created , The query performance of tables is greatly improved , But the performance of writing will also be greatly reduced 

 Don't create indexes randomly ！！！

b+ Trees

 Only leaf nodes are real data , The data stored in other nodes is virtual data , It's only used to show the way 

 The higher the tree level, the more steps it takes to query data （ There are several layers in the tree , It takes several layers to query data ）

 A disk block storage is limited , When storing, I hope that the larger the data stored, the better , So recommend  id  A primary key , because   Integer takes up less space, and a disk block stores more data , Lowered the height of the tree , Thus reducing the number of queries 

Clustered index (primary key)

#  A clustered index is   Primary key 

''' innodb  There are only two documents   Directly set the primary key   Stored in data table （idb） MyIsam  Three files   There will be a file to search for alone  '''

Secondary index （unique key）

'''  Can't you use the primary key all the time when querying data , Other fields may also be used , There is no way to use clustered indexes , At this time, you can set the auxiliary index for other fields according to （ It's also  b+ Trees ）  The secondary index leaf node stores the primary key value corresponding to the data   First get the primary key value of the data according to the auxiliary index   After that, you still need to query the data in the clustered index of the primary key  '''

Overlay index

The leaf node of the secondary index has obtained the required data

#  to  name  Set secondary index 
select name from user where name = "aoteman";

#  Non coverage index 
select age from user where name = "aoteman"