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MySQL learning notes (Advanced)

2022-07-02 20:57:00 【Love and dream】

One 、MySQL Stored procedures and functions

1. The concept of stored procedures and functions

Stored procedures and functions are A segment that has been compiled in advance and stored in a database SQL Collection of statements

2. The benefits of stored procedures and functions

Stored procedures and functions can be reused , Reduce the workload of developers . Be similar to java Methods in can be called multiple times
Reduce network traffic , Stored procedures and functions are on the server , Just pass the name and parameters when calling
Reduce data transfer between database and application server , It can improve the efficiency of data processing
Implement some business logic at the database level , It can reduce the business processing at the code level

3. The difference between stored procedures and functions

Function must have a return value
The stored procedure did not return a value

4. Create stored procedure

Little knowledge

/*  This keyword is used to declare sql Statement separator , tell MySQL The command is over ！ sql The default separator for a statement is a semicolon , But sometimes we need a function sql The statement contains a semicolon , But not as an end sign .  It can be used at this time DELIMITER To specify the separator ！ */
--  Standard grammar 
DELIMITER  Separator

Data preparation

--  establish db8 database 
CREATE DATABASE db8;

--  Use db8 database 
USE db8;

--  Create student table 
CREATE TABLE student(
	id INT PRIMARY KEY AUTO_INCREMENT,	--  Student id
	NAME VARCHAR(20),					--  The student's name 
	age INT,							--  Student age 
	gender VARCHAR(5),					--  Student gender 
	score INT                           --  Student achievement 
);
--  Add data 
INSERT INTO student VALUES (NULL,' Zhang San ',23,' male ',95),(NULL,' Li Si ',24,' male ',98),
(NULL,' Wang Wu ',25,' Woman ',100),(NULL,' Zhao Liu ',26,' Woman ',90);

--  Group by gender , Check the total score of each group . In ascending order of total scores 
SELECT gender,SUM(score) getSum FROM student GROUP BY gender ORDER BY getSum ASC;

Create stored procedure syntax

  --  Change the separator to $
    DELIMITER $
    
    --  Standard grammar 
    CREATE PROCEDURE  Stored procedure name ( Parameters ...)
    BEGIN
    	sql sentence ;
    END$
    
    --  Change the separator to semicolon 
    DELIMITER ;

-  Create stored procedure 

    --  Change the separator to $
    DELIMITER $
    
    --  Create stored procedure , Package group query student total score sql sentence 
    CREATE PROCEDURE stu_group()
    BEGIN
    	SELECT gender,SUM(score) getSum FROM student GROUP BY gender ORDER BY getSum ASC;
    END$
    
    --  Change the separator to semicolon 
    DELIMITER ;

5. Calling stored procedure

Call stored procedure syntax

--  Standard grammar 
CALL  Stored procedure name ( The actual parameter );

--  call stu_group stored procedure 
CALL stu_group();

6. View stored procedures

Look at the stored procedure syntax

--  Query all stored procedures in the database   Standard grammar 
SELECT * FROM mysql.proc WHERE db=' Database name ';

7. Delete stored procedure

Delete stored procedure syntax

   --  Standard grammar 
    DROP PROCEDURE [IF EXISTS]  Stored procedure name ;
    
    --  Delete stu_group stored procedure 
    DROP PROCEDURE stu_group;

8. Stored procedure syntax

8.1 Introduction to stored procedure syntax

Stored procedures are programmable . It means that variables can be used 、 expression 、 Conditional control statements 、 Loop statement, etc , To complete more complex functions ！

8.2 Use of variables

Defining variables

 --  Standard grammar 
    DECLARE  Variable name   data type  [DEFAULT  The default value is ];
    --  Be careful ： DECLARE Local variables are defined , Can only be used in BEGIN END Within limits 
    
    --  Define a int Type variable 、 And the default value is 10
    DELIMITER $
    
    CREATE PROCEDURE pro_test1()
    BEGIN
    	DECLARE num INT DEFAULT 10;   --  Defining variables 
    	SELECT num;                   --  Query variables 
    END$
    
    DELIMITER ;
    
    --  call pro_test1 stored procedure 
    CALL pro_test1();

The assignment of a variable 1

   --  Standard grammar 
    SET  Variable name  =  A variable's value ;
    
    --  Define string type variables , And the assignment 
    DELIMITER $
    
    CREATE PROCEDURE pro_test2()
    BEGIN
    	DECLARE NAME VARCHAR(10);   --  Defining variables 
    	SET NAME = ' stored procedure ';       --  Assign values to variables 
    	SELECT NAME;                --  Query variables 
    END$
    
    DELIMITER ;
    
    --  call pro_test2 stored procedure 
    CALL pro_test2();

The assignment of a variable 2

   --  Standard grammar 
    SELECT  Name  INTO  Variable name  FROM  Table name  [WHERE  Conditions ];
    
    --  Define two int Variable , It's used to store the total scores of boys and girls 
    DELIMITER $
    
    CREATE PROCEDURE pro_test3()
    BEGIN
    	DECLARE men,women INT;  --  Defining variables 
    	SELECT SUM(score) INTO men FROM student WHERE gender=' male ';    --  Calculate the total score of male students and assign it to men
    	SELECT SUM(score) INTO women FROM student WHERE gender=' Woman ';  --  Calculate the total score of female students and assign it to women
    	SELECT men,women;           --  Query variables 
    END$
    
    DELIMITER ;
    
    --  call pro_test3 stored procedure 
    CALL pro_test3();

8.3if Use of statements

Standard grammar

 --  Standard grammar 
    IF  Judge the condition 1 THEN  Executive sql sentence 1;
    [ELSEIF  Judge the condition 2 THEN  Executive sql sentence 2;]
    ...
    [ELSE  Executive sql sentence n;]
    END IF;

Case presentation

   /*  Define a int Variable , It is used to store the total grade of the class   Define a varchar Variable , Used to store score descriptions   According to the total score ： 380 Above and above   Study well  320 ~ 380  Learning is good  320 following   General learning  */
    DELIMITER $
    
    CREATE PROCEDURE pro_test4()
    BEGIN
    	--  Define the total score variable 
    	DECLARE total INT;
    	--  Define the score to describe the variable 
    	DECLARE description VARCHAR(10);
    	--  Assign a value to the total score variable 
    	SELECT SUM(score) INTO total FROM student;
    	--  Judge the total score 
    	IF total >= 380 THEN 
    		SET description = ' Study well ';
    	ELSEIF total >= 320 AND total < 380 THEN 
    		SET description = ' Learning is good ';
    	ELSE 
    		SET description = ' General learning ';
    	END IF;
    	
    	--  Query the total score and description information 
    	SELECT total,description;
    END$
    
    DELIMITER ;
    
    --  call pro_test4 stored procedure 
    CALL pro_test4();

8.4 Parameter passing

The syntax of parameter passing

  DELIMITER $
    
    --  Standard grammar 
    CREATE PROCEDURE  Stored procedure name ([IN|OUT|INOUT]  Parameter name   data type )
    BEGIN
    	 Executive sql sentence ;
    END$
    /* IN: Represents the input parameter , The actual data needs to be passed by the caller . default  OUT: Represents the output parameter , This parameter can be used as the return value  INOUT: It can be used as an input parameter , It can also be used as an output parameter  */
    DELIMITER ;

Input parameters
- Standard grammar

   DELIMITER $
      
      --  Standard grammar 
      CREATE PROCEDURE  Stored procedure name (IN  Parameter name   data type )
      BEGIN
      	 Executive sql sentence ;
      END$
      
      DELIMITER ;

Case presentation

   /*  Enter the total score variable , Represents the student's total score   Define a varchar Variable , Used to store score descriptions   According to the total score ： 380 Above and above   Study well  320 ~ 380  Learning is good  320 following   General learning  */
      DELIMITER $
      
      CREATE PROCEDURE pro_test5(IN total INT)
      BEGIN
      	--  Define the score to describe the variable 
      	DECLARE description VARCHAR(10);
      	--  Judge the total score 
      	IF total >= 380 THEN 
      		SET description = ' Study well ';
      	ELSEIF total >= 320 AND total < 380 THEN 
      		SET description = ' Learning is good ';
      	ELSE 
      		SET description = ' General learning ';
      	END IF;
      	
      	--  Query the total score and description information 
      	SELECT total,description;
      END$
      
      DELIMITER ;
      
      --  call pro_test5 stored procedure 
      CALL pro_test5(390);
      CALL pro_test5((SELECT SUM(score) FROM student));

Output parameters
- Standard grammar

DELIMITER $
  
  --  Standard grammar 
  CREATE PROCEDURE  Stored procedure name (OUT  Parameter name   data type )
  BEGIN
  	 Executive sql sentence ;
  END$
  
  DELIMITER ;

Case presentation

  /*  Enter the total score variable , Represents the student's total score   The output fraction describes the variable , A description of the student's total score   According to the total score ： 380 Above and above   Study well  320 ~ 380  Learning is good  320 following   General learning  */
  DELIMITER $
  
  CREATE PROCEDURE pro_test6(IN total INT,OUT description VARCHAR(10))
  BEGIN
  	--  Judge the total score 
  	IF total >= 380 THEN 
  		SET description = ' Study well ';
  	ELSEIF total >= 320 AND total < 380 THEN 
  		SET description = ' Learning is good ';
  	ELSE 
  		SET description = ' General learning ';
  	END IF;
  END$
  
  DELIMITER ;
  
  --  call pro_test6 stored procedure 
  CALL pro_test6(310,@description);
  
  --  Query the total score description 
  SELECT @description;

Little knowledge

  @ Variable name :   This variable should be preceded by the name of the variable “@” Symbol , It's called the user session variable , On behalf of the whole conversation, he is useful , This is similar to a global variable .
  
  @@ Variable name :  This adds... To the variable  "@@"  Symbol ,  It's called system variables

8.5 case Use of statements

Standard grammar 1

--  Standard grammar 
CASE  expression 
WHEN  value 1 THEN  perform sql sentence 1;
[WHEN  value 2 THEN  perform sql sentence 2;]
...
[ELSE  perform sql sentence n;]
END CASE;

Standard grammar 2

--  Standard grammar 
CASE
WHEN  Judge the condition 1 THEN  perform sql sentence 1;
[WHEN  Judge the condition 2 THEN  perform sql sentence 2;]
...
[ELSE  perform sql sentence n;]
END CASE;

Case presentation

  /*  Enter the total score variable , Represents the student's total score   Define a varchar Variable , Used to store score descriptions   According to the total score ： 380 Above and above   Study well  320 ~ 380  Learning is good  320 following   General learning  */
    DELIMITER $
    
    CREATE PROCEDURE pro_test7(IN total INT)
    BEGIN
    	--  Defining variables 
    	DECLARE description VARCHAR(10);
    	--  Use case Judge 
    	CASE
    	WHEN total >= 380 THEN
    		SET description = ' Study well ';
    	WHEN total >= 320 AND total < 380 THEN
    		SET description = ' Learning is good ';
    	ELSE 
    		SET description = ' General learning ';
    	END CASE;
    	
    	--  Query score description information 
    	SELECT description;
    END$
    
    DELIMITER ;
    
    --  call pro_test7 stored procedure 
    CALL pro_test7(390);
    CALL pro_test7((SELECT SUM(score) FROM student));

8.6 while loop

Standard grammar

  --  Standard grammar 
     Initialization statement ;
    WHILE  Conditional statements  DO
    	 Loop body statement ;
    	 Conditional control statements ;
    END WHILE;

Case presentation

   /*  Calculation 1~100 Between even numbers and  */
    DELIMITER $
    
    CREATE PROCEDURE pro_test8()
    BEGIN
    	--  Define the summation variable 
    	DECLARE result INT DEFAULT 0;
    	--  Define initialization variables 
    	DECLARE num INT DEFAULT 1;
    	-- while loop 
    	WHILE num <= 100 DO
    		--  Even number judgment 
    		IF num%2=0 THEN
    			SET result = result + num; --  Add up 
    		END IF;
    		
    		--  Give Way num+1
    		SET num = num + 1;         
    	END WHILE;
    	
    	--  Query the sum result 
    	SELECT result;
    END$
    
    DELIMITER ;
    
    --  call pro_test8 stored procedure 
    CALL pro_test8();

8.7 repeat loop

Standard grammar

  --  Standard grammar 
     Initialization statement ;
    REPEAT
    	 Loop body statement ;
    	 Conditional control statements ;
    	UNTIL  Conditional statements 
    END REPEAT;
    
    --  Be careful ：repeat The loop stops when the condition is met .while Loop is executed when the condition is met

Case presentation

   /*  Calculation 1~10 The sum between  */
    DELIMITER $
    
    CREATE PROCEDURE pro_test9()
    BEGIN
    	--  Define the summation variable 
    	DECLARE result INT DEFAULT 0;
    	--  Define initialization variables 
    	DECLARE num INT DEFAULT 1;
    	-- repeat loop 
    	REPEAT
    		--  Add up 
    		SET result = result + num;
    		--  Give Way num+1
    		SET num = num + 1;
    		
    		--  Stop the cycle 
    		UNTIL num>10
    	END REPEAT;
    	
    	--  Query the sum result 
    	SELECT result;
    END$
    
    DELIMITER ;
    
    --  call pro_test9 stored procedure 
    CALL pro_test9();

8.8 loop loop

Standard grammar

--  Standard grammar 
 Initialization statement ;
[ Cycle name :] LOOP
	 Conditional statements 
		[LEAVE  Cycle name ;]
	 Loop body statement ;
	 Conditional control statements ;
END LOOP  Cycle name ;

--  Be careful ：loop Simple loops can be implemented , But exiting the loop requires other statements to define . We can use leave Sentence completion ！
--  If you don't add a statement to exit the loop , Then it becomes an endless cycle .

Case presentation

  /*  Calculation 1~10 The sum between  */
    DELIMITER $
    
    CREATE PROCEDURE pro_test10()
    BEGIN
    	--  Define the summation variable 
    	DECLARE result INT DEFAULT 0;
    	--  Define initialization variables 
    	DECLARE num INT DEFAULT 1;
    	-- loop loop 
    	l:LOOP
    		--  Conditions established , Stop the cycle 
    		IF num > 10 THEN
    			LEAVE l;
    		END IF;
    	
    		--  Add up 
    		SET result = result + num;
    		--  Give Way num+1
    		SET num = num + 1;
    	END LOOP l;
    	
    	--  Query the sum result 
    	SELECT result;
    END$
    
    DELIMITER ;
    
    --  call pro_test10 stored procedure 
    CALL pro_test10();

8.9 The cursor

The concept of cursors
- Cursors can traverse multiple rows of returned results , Get a whole line of data at a time
- In stored procedures and functions, cursors can be used to cycle the result set
- In simple terms, cursors are similar to iterator traversal of a collection
- MySQL Cursors in can only be used in stored procedures and functions
The syntax of cursors
- Create cursors

 --  Standard grammar 
  DECLARE  Cursor name  CURSOR FOR  Inquire about sql sentence ;

Open cursor

 --  Standard grammar 
  OPEN  Cursor name ;

Get data using cursors

  --  Standard grammar 
  FETCH  Cursor name  INTO  Variable name 1, Variable name 2,...;

Close cursor

  --  Standard grammar 
  CLOSE  Cursor name ;

Basic use of cursors

--  establish stu_score surface 
CREATE TABLE stu_score(
	id INT PRIMARY KEY AUTO_INCREMENT,
	score INT
);

/*  take student Save all scores in the table to stu_score In the table  */
DELIMITER $

CREATE PROCEDURE pro_test11()
BEGIN
	--  Define performance variables 
	DECLARE s_score INT;
	--  Create cursors , Query all student achievement data 
	DECLARE stu_result CURSOR FOR SELECT score FROM student;
	
	--  Open cursor 
	OPEN stu_result;
	
	--  Use cursors , Ergodic result , Get the number 1 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Use cursors , Ergodic result , Get the number 2 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Use cursors , Ergodic result , Get the number 3 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Use cursors , Ergodic result , Get the number 4 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Close cursor 
	CLOSE stu_result;
END$

DELIMITER ;

--  call pro_test11 stored procedure 
CALL pro_test11();

--  Inquire about stu_score surface 
SELECT * FROM stu_score;


-- ===========================================================
/*  What happened ： student There is a total of 4 Data , We traversed the cursor 4 Time , No problem ！  But how many times do you traverse the cursor ？ There will be problems  */
DELIMITER $

CREATE PROCEDURE pro_test11()
BEGIN
	--  Define performance variables 
	DECLARE s_score INT;
	--  Create cursors , Query all student achievement data 
	DECLARE stu_result CURSOR FOR SELECT score FROM student;
	
	--  Open cursor 
	OPEN stu_result;
	
	--  Use cursors , Ergodic result , Get the number 1 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Use cursors , Ergodic result , Get the number 2 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Use cursors , Ergodic result , Get the number 3 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Use cursors , Ergodic result , Get the number 4 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Use cursors , Ergodic result , Get the number 5 Row data 
	FETCH stu_result INTO s_score;
	--  Save data to stu_score In the table 
	INSERT INTO stu_score VALUES (NULL,s_score);
	
	--  Close cursor 
	CLOSE stu_result;
END$

DELIMITER ;

--  call pro_test11 stored procedure 
CALL pro_test11();

--  Inquire about stu_score surface , Although the data are correct , However, an error will be reported when executing the stored procedure 
SELECT * FROM stu_score;

Optimized use of cursors ( In combination with recycling )

/*  When the cursor ends , Will trigger the cursor end event . We can use this feature to complete the loop operation   Tagging thought ： 1. Define a variable , The default value is 0( It means there's data ) 2. When the cursor ends , Change the variable value to 1( Means there's no data ) */
-- 1. Define a variable , The default value is 0( It means there's data )
DECLARE flag INT DEFAULT 0;
-- 2. When the cursor ends , Change the variable value to 1( Means there's no data )
DECLARE EXIT HANDLER FOR NOT FOUND SET flag = 1;

/*  take student Save all scores in the table to stu_score In the table  */
DELIMITER $

CREATE PROCEDURE pro_test12()
BEGIN
	--  Define performance variables 
	DECLARE s_score INT;
	--  Define tag variables 
	DECLARE flag INT DEFAULT 0;
	--  Create cursors , Query all student achievement data 
	DECLARE stu_result CURSOR FOR SELECT score FROM student;
	--  At the end of the cursor , Change the tag variable to 1
	DECLARE EXIT HANDLER FOR NOT FOUND SET flag = 1;
	
	--  Open cursor 
	OPEN stu_result;
	
	--  Recycle cursors 
	REPEAT
		--  Use cursors , Ergodic result , Get the data 
		FETCH stu_result INTO s_score;
		--  Save data to stu_score In the table 
		INSERT INTO stu_score VALUES (NULL,s_score);
	UNTIL flag=1
	END REPEAT;
	
	--  Close cursor 
	CLOSE stu_result;
END$

DELIMITER ;

--  call pro_test12 stored procedure 
CALL pro_test12();

--  Inquire about stu_score surface 
SELECT * FROM stu_score;

9. Summary of stored procedures

The stored procedure is A segment that has been compiled in advance and stored in a database SQL Collection of statements . You can do some business processing at the database level
To put it bluntly, a stored procedure is to put sql Statements are encapsulated as methods , You can then call the method to execute sql Just a statement
Benefits of stored procedures
- Security
- Efficient
- Strong reusability

10. Storage function

Stored functions and stored procedures are very similar . What storage functions can do , Stored procedures can also do ！
The storage function has a return value , The stored procedure did not return a value ( Parametric out In fact, it is equivalent to returning data )
Standard grammar
- Create a storage function

 DELIMITER $
  
  --  Standard grammar 
  CREATE FUNCTION  The name of the function ([ Parameters   data type ])
  RETURNS  return type 
  BEGIN
  	 Executive sql sentence ;
  	RETURN  result ;
  END$
  
  DELIMITER ;

Calling the storage function

  --  Standard grammar 
  SELECT  The name of the function ( The actual parameter );

Delete storage function

  --  Standard grammar 
  DROP FUNCTION  The name of the function ;

Case presentation

   /*  Define storage functions , Get the score in the student table greater than 95 The number of students divided  */
    DELIMITER $
    
    CREATE FUNCTION fun_test1()
    RETURNS INT
    BEGIN
    	--  Define statistical variables 
    	DECLARE result INT;
    	--  The query result is greater than 95 The number of students divided , Assign values to statistical variables 
    	SELECT COUNT(*) INTO result FROM student WHERE score > 95;
    	--  Return Statistics 
    	RETURN result;
    END$
    
    DELIMITER ;
    
    --  call fun_test1 Storage function 
    SELECT fun_test1();

Two 、MySQL trigger

1. The concept of trigger

Triggers are database objects related to tables , Can be in insert/update/delete Before or after , Trigger and execute the... Defined in the trigger SQL sentence . This feature of trigger can help to ensure the integrity of data in database 、 logging 、 Data verification and other operations .
Use the alias NEW and OLD To refer to the changed record content in the trigger , This is similar to other databases . Now triggers only support row level triggering , Statement level triggering is not supported .

Trigger Type	OLD The meaning of	NEW The meaning of
INSERT Type trigger	nothing ( Because there is no data in the state before insertion )	NEW Indicates the data to be added or added
UPDATE Type trigger	OLD Represents the data before modification	NEW Represents the data that will be or has been modified
DELETE Type trigger	OLD Data that will be or has been deleted	nothing ( Because there is no data after deletion )

2. Create trigger

Standard grammar

 DELIMITER $
    
    CREATE TRIGGER  Trigger Name 
    BEFORE|AFTER INSERT|UPDATE|DELETE
    ON  Table name 
    [FOR EACH ROW]  --  Line level triggers 
    BEGIN
    	 The function that the trigger performs ;
    END$
    
    DELIMITER ;

Trigger Demo . Record the data change log of account table through trigger . contain ： increase 、 modify 、 Delete
- Create account table

  --  establish db9 database 
  CREATE DATABASE db9;
  
  --  Use db9 database 
  USE db9;
  
  --  Create account table account
  CREATE TABLE account(
  	id INT PRIMARY KEY AUTO_INCREMENT,	--  Account id
  	NAME VARCHAR(20),					--  full name 
  	money DOUBLE						--  balance 
  );
  --  Add data 
  INSERT INTO account VALUES (NULL,' Zhang San ',1000),(NULL,' Li Si ',2000);

Create a log table

   --  Create a log table account_log
      CREATE TABLE account_log(
      	id INT PRIMARY KEY AUTO_INCREMENT,	--  journal id
      	operation VARCHAR(20),				--  Operation type  (insert update delete)
      	operation_time DATETIME,			--  Operating time 
      	operation_id INT,					--  Operation table id
      	operation_params VARCHAR(200)       --  Operating parameters 
      );

establish INSERT trigger

 --  establish INSERT trigger 
  DELIMITER $
  
  CREATE TRIGGER account_insert
  AFTER INSERT
  ON account
  FOR EACH ROW
  BEGIN
  	INSERT INTO account_log VALUES (NULL,'INSERT',NOW(),new.id,CONCAT(' After inserting {id=',new.id,',name=',new.name,',money=',new.money,'}'));
  END$
  
  DELIMITER ;
  
  --  towards account Table add record 
  INSERT INTO account VALUES (NULL,' Wang Wu ',3000);
  
  --  Inquire about account surface 
  SELECT * FROM account;
  
  --  Query log table 
  SELECT * FROM account_log;

establish UPDATE trigger
– establish UPDATE trigger

   DELIMITER $
      
      CREATE TRIGGER account_update
      AFTER UPDATE
      ON account
      FOR EACH ROW
      BEGIN
      	INSERT INTO account_log VALUES (NULL,'UPDATE',NOW(),new.id,CONCAT(' Before the change {id=',old.id,',name=',old.name,',money=',old.money,'}',' After modification {id=',new.id,',name=',new.name,',money=',new.money,'}'));
      END$
      
      DELIMITER ;
      
      --  modify account surface 
      UPDATE account SET money=3500 WHERE id=3;
      
      --  Inquire about account surface 
      SELECT * FROM account;
      
      --  Query log table 
      SELECT * FROM account_log;
  -  establish DELETE trigger 
      --  establish DELETE trigger 
      DELIMITER $
      
      CREATE TRIGGER account_delete
      AFTER DELETE
      ON account
      FOR EACH ROW
      BEGIN
      	INSERT INTO account_log VALUES (NULL,'DELETE',NOW(),old.id,CONCAT(' Before deleting {id=',old.id,',name=',old.name,',money=',old.money,'}'));
      END$
      
      DELIMITER ;
      
      --  Delete account Table data 
      DELETE FROM account WHERE id=3;
      
      --  Inquire about account surface 
      SELECT * FROM account;
      
      --  Query log table 
      SELECT * FROM account_log;

3. Check triggers

--  Standard grammar 
SHOW TRIGGERS;

--  Check triggers 
SHOW TRIGGERS;

4. Delete trigger

  --  Standard grammar 
    DROP TRIGGER  Trigger Name ;
    
    --  Delete DELETE trigger 
    DROP TRIGGER account_delete;

5. Summary of triggers

Triggers are database objects related to tables
Can be in insert/update/delete Before or after , Trigger and execute the... Defined in the trigger SQL sentence
This feature of trigger can help to ensure the integrity of data in database 、 logging 、 Data verification and other operations
Use the alias NEW and OLD To refer to the changed record content in the trigger

3、 ... and 、MySQL Business

1. Concept of transactions

One or more SQL Statements form an execution unit , The feature is that this unit either succeeds or fails at the same time , Each of the units SQL Statements depend on each other , To form a whole , If one of the SQL Statement execution failed or an error occurred , Then the whole unit will roll back , Back to the original state of the transaction , If all of the units SQL Statements are executed successfully , Then the transaction goes smoothly .

2. Data preparation for transactions

--  establish db10 database 
CREATE DATABASE db10;

--  Use db10 database 
USE db10;

--  Create account table 
CREATE TABLE account(
	id INT PRIMARY KEY AUTO_INCREMENT,	--  Account id
	NAME VARCHAR(20),			--  title of account 
	money DOUBLE				--  The account balance 
);
--  Add data 
INSERT INTO account VALUES (NULL,' Zhang San ',1000),(NULL,' Li Si ',1000);

3. Unmanaged transaction Demo

 --  Zhang San transfers money to Li Si 500 element 
    -- 1. Zhang San's account -500
    UPDATE account SET money=money-500 WHERE NAME=' Zhang San ';
    -- 2. Li Si's account +500
     Something went wrong ...
    UPDATE account SET money=money+500 WHERE NAME=' Li Si ';
    
    --  In this scenario , These two sql The statement either succeeds at the same time , Or fail at the same time . It needs to be managed by the firm ！

4. Management transaction Demo

The three steps of operating a transaction
1. Open transaction ： Record the rollback point , And notify the server , A set of operations will be performed , Or at the same time 、 Or fail at the same time
2. perform sql sentence ： Carry out one or more specific sql sentence
3. End the business ( Submit | Roll back )
  - Submit ： No problem , Update the data
  - Roll back ： Problems arise , The data is restored to the state when the transaction was opened
Open transaction

--  Standard grammar 
START TRANSACTION;

Roll back the transaction

  --  Standard grammar 
    ROLLBACK;

Commit transaction

  --  Standard grammar 
    COMMIT;

Management transaction Demo

  --  Open transaction 
    START TRANSACTION;
    
    --  Zhang San transfers money to Li Si 500 element 
    -- 1. Zhang San's account -500
    UPDATE account SET money=money-500 WHERE NAME=' Zhang San ';
    -- 2. Li Si's account +500
    --  Something went wrong ...
    UPDATE account SET money=money+500 WHERE NAME=' Li Si ';
    
    --  Roll back the transaction ( Problems arise )
    ROLLBACK;
    
    --  Commit transaction ( No problem )
    COMMIT;

5. How to commit a transaction

submission
- Automatic submission (MySQL The default is auto submit )
- Manual submission
Modify submission method
- See how to submit

 --  Standard grammar 
  SELECT @@AUTOCOMMIT;  -- 1 On behalf of auto submit  0 On behalf of hand submit

Modify submission method

--  Standard grammar 
  SET @@AUTOCOMMIT= Numbers ;
  
  --  Change to manual submit 
  SET @@AUTOCOMMIT=0;
  
  --  See how to submit 
  SELECT @@AUTOCOMMIT;

6. The four characteristics of affairs (ACID)

Atomicity (atomicity)
- Atomicity refers to the success of all operations contained in a transaction , Either all failures roll back , Therefore, if the transaction operation succeeds, it must be fully applied to the database , If the operation fails, there will be no impact on the database
Uniformity (consistency)
- Consistency refers to the fact that a transaction must transform a database from one consistency state to another , That is to say, a transaction must be in a consistent state before and after execution
- Take money transfer , Suppose the sum of Zhang San's and Li Si's money is 2000, So no matter A and B How to transfer money between banks , Transfer several times , After the transaction, the sum of money of the two users should be 2000, This is the consistency of transactions
Isolation, (isolcation)
- Isolation is when multiple users access the database concurrently , For example, when operating the same table , A transaction opened by a database for each user , It cannot be interfered by the operation of other transactions , Multiple concurrent transactions should be isolated from each other
persistence (durability)
- Persistence means that once a transaction is committed , So the change to the data in the database is permanent , Even in the case of database system failure, the transaction commit operation will not be lost

7. The isolation level of the transaction

The concept of isolation level
- When multiple clients operate , Transactions between clients should be isolated , Mutually independent , Unaffected .
- If multiple transactions operate on the same batch of data , You need to set different isolation levels , Otherwise, there will be problems .
- Let's take a look at the names of the four isolation levels , Let's look at the possible problems
Four levels of isolation
1 Read uncommitted read uncommitted
2 Read submitted read committed
3 Repeatable repeatable read
4 Serialization serializable
Possible problems

problem	The phenomenon
Dirty reading	Read data from another uncommitted transaction during one transaction , The results of the two queries are inconsistent
It can't be read repeatedly	It means that the data modified and committed in another transaction is read in the process of one transaction , The results of the two queries are inconsistent
Fantasy reading	select Whether a record exists , non-existent , Ready to insert this record , But enforcement insert Found that this record already exists , Can't insert . Or there is no execution delete Delete , I found that the deletion was successful

Query database isolation level

--  Standard grammar 
SELECT @@TX_ISOLATION;

Modify the database isolation level

--  Standard grammar 
SET GLOBAL TRANSACTION ISOLATION LEVEL  Level string ;

--  Modify the database isolation level to read uncommitted
SET GLOBAL TRANSACTION ISOLATION LEVEL read uncommitted;

--  Check the isolation level 
SELECT @@TX_ISOLATION;   --  It needs to be disconnected after modification

8. Transaction isolation level demonstration

The problem of dirty reading
- window 1

 --  Query account table 
  select * from account;
  
  --  Set the isolation level to read uncommitted
  set global transaction isolation level read uncommitted;
  
  --  Open transaction 
  start transaction;
  
  --  Transfer accounts 
  update account set money = money - 500 where id = 1;
  update account set money = money + 500 where id = 2;
  
  --  window 2 Query transfer results  , Dirty reading ( Query uncommitted data of other transactions )
  
  --  window 2 After checking the transfer results , rollback 
  rollback;

window 2

  --  Query isolation level 
  select @@tx_isolation;
  
  --  Open transaction 
  start transaction;
  
  --  Query account table 
  select * from account;

Solve the problem of dirty reading and demonstrate the problem of non repeatable reading
- window 1

 --  Set the isolation level to read committed
  set global transaction isolation level read committed;
  
  --  Open transaction 
  start transaction;
  
  --  Transfer accounts 
  update account set money = money - 500 where id = 1;
  update account set money = money + 500 where id = 2;
  
  --  window 2 Check the transfer results , Nothing has changed ( The dirty reading problem has been solved )
  
  --  Perform commit transactions .
  commit;
  
  --  window 2 Check the transfer results , The data has changed ( There's the problem of not being able to read again , Read committed data from other transactions )

window 2

     --  Query isolation level 
      select @@tx_isolation;
      
      --  Open transaction 
      start transaction;
      
      --  Query account table 
      select * from account;
-  Solve the problem of non repeatable reading 
  -  window 1
      --  Set the isolation level to repeatable read
      set global transaction isolation level repeatable read;
      
      --  Open transaction 
      start transaction;
      
      --  Transfer accounts 
      update account set money = money - 500 where id = 1;
      update account set money = money + 500 where id = 2;
      
      --  window 2 Check the transfer results , Nothing has changed 
      
      --  Perform commit transactions 
      commit;
      
      --  At this time, the window 2 As long as it's still in the last business , The results are the same . Only the window 2 End the business , To see the change ( The problem of non repeatable reading has been solved )

window 2

 --  Query isolation level 
  select @@tx_isolation;
  
  --  Open transaction 
  start transaction;
  
  --  Query account table 
  select * from account;
  
  --  Commit transaction 
  commit;
  
  --  Query account table 
  select * from account;

The problem and solution of unreal reading
- window 1

  --  Set the isolation level to repeatable read
  set global transaction isolation level repeatable read;
  
  --  Open transaction 
  start transaction;
  
  --  Add a record 
  INSERT INTO account VALUES (3,' Wang Wu ',1500);
  
  --  Query account table , In this window, you can see id by 3 Result 
  SELECT * FROM account;
  
  --  Commit transaction 
  COMMIT;

window 2

 --  Query isolation level 
  select @@tx_isolation;
  
  --  Open transaction 
  start transaction;
  
  --  Query account table , The newly added id by 3 The record of 
  select * from account;
  
  --  add to id by 3 A piece of data , Discovery add failed . There's unreal reading 
  INSERT INTO account VALUES (3,' test ',200);
  
  --  Commit transaction 
  COMMIT;
  
  --  Query account table , The newly added id by 3 The record of 
  select * from account;

Solve the problem of unreal reading

  /*  window 1 */
  --  Set the isolation level to serializable
  set global transaction isolation level serializable;
  
  --  Open transaction 
  start transaction;
  
  --  Add a record 
  INSERT INTO account VALUES (4,' Zhao Liu ',1600);
  
  --  Query account table , In this window, you can see id by 4 Result 
  SELECT * FROM account;
  
  --  Commit transaction 
  COMMIT;

  /*  window 2 */
  --  Query isolation level 
  select @@tx_isolation;
  
  --  Open transaction 
  start transaction;
  
  --  Query account table , The query statement cannot be executed , The data table is locked ！ Only the window 1 After committing the transaction , Before you can continue to operate 
  select * from account;
  
  --  add to id by 4 A piece of data , It's been discovered that , No more ！ The problem of unreal reading has been solved 
  INSERT INTO account VALUES (4,' test ',200);
  
  --  Commit transaction 
  COMMIT;

9. Isolation level summary

Isolation level	name	Dirty reading	Non repeatable reading appears	Phantom reading	Database default isolation level
1	read uncommitted	Read uncommitted	yes	yes	yes
2	read committed	Read submitted	no	yes	yes Oracle / SQL Server
3	repeatable read	Repeatable	no	no	yes MySQL
4	serializable	Serialization	no	no	no

Be careful ： The isolation level is getting higher and higher from small to large , But the efficiency is getting lower , So it's not recommended READ UNCOMMITTED and SERIALIZABLE Isolation level .

10. A summary of the business

One or more SQL Statements form an execution unit , The feature is that this unit either succeeds or fails at the same time . For example, transfer operations
Open transaction ：start transaction;
Roll back the transaction ：rollback;
Commit transaction ：commit;
Four characteristics of business
- Atomicity
- persistence
- Isolation,
- Uniformity
The isolation level of the transaction
- read uncommitted( Read uncommitted )
- read committed ( Read submitted )
- repeatable read ( Repeatable )
- serializable ( Serialization )

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