MySQL completely disintegrated 19: cursor related

itread01 2021-01-21 20:07:12
mysql completely disintegrated cursor related


Define

We often come across a situation like this , We need to traverse the results of our query , And to traverse to each data processing , In this case, the cursor will be used .
therefore : Cursor (Cursor) It's a way of processing data stored in MySQL Database query method on server , To view or process the data in the result set , Provides the ability to traverse data one line at a time in the result set .
Cursors are mainly used for looping 、 Storage process 、 Function 、 Trigger in .

The function of the cursor

Like the one above us students Students , You need to traverse each user , Then add or subtract points based on their other evaluations . At this time, we need to query all the students' information ( Include grades ).
1 select studentid,studentname,score from students; 
After execution, the returned student data collection , If we need to traverse the student data one by one , Then add points according to the specific situation , Then you need to use a cursor .
A cursor is equivalent to an index , This indicator points to select The first line of information , You can traverse the following data by moving indicators . 

The use of cursors

Declaration cursor : Create a cursor , And specify what the cursor needs to traverse select Inquire about , You don't do this when you declare a cursor sql.
Open cursor : When you open the cursor , Will execute the cursor corresponding to select Sentence .
Traversal data : Use cursor loop traversal select Each row of data in the result , And then deal with it .
Business operations : The process of operating on each line of data traversed , You can place any executed statement that needs to be executed ( Add, delete, and check ): It depends here
Close cursor : The cursor must be released after use .
notes : The temporary fields used need to be declared before the cursor is defined .

Declaration cursor

1 DECLARE cursor_name CURSOR FOR select_statement; 
Declare a cursor . You can also define multiple cursors in a subroutine , But every cursor in a block must have a unique name . After declaring a cursor, it is also a single operation , however SELECT A statement cannot have INTO Clause .
One begin end Only one cursor can be declared in .

Open cursor

1 OPEN cursor_name; 
Open previously declared cursor .

Traverse cursor data

1 FETCH cursor_name INTO var_list;
This statement reads the next line with the specified open cursor ( If there's a next line ), And forward cursor indicator . Take the result of the current line , Put the result in the corresponding variable , And point the cursor index to the next line of data .
When calling fetch When , Will get the data of the current row , If the current row has no data , Will trigger mysql Internal NOT FOUND Mistake .

Close cursor

1 CLOSE cursor_name; 
Remember to close the cursor after use .

For example

Write a function , It contains the right students The calculation of students' grades and additional points
Data base
 1 mysql> select * from students; 2 +-----------+-------------+-------+---------+ 3 | studentid | studentname | score | classid | 4 +-----------+-------------+-------+---------+ 5 | 1 | brand | 97.5 | 1 | 6 | 2 | helen | 96.5 | 1 | 7 | 3 | lyn | 96 | 1 | 8 | 4 | sol | 97 | 1 | 9 | 5 | b1 | 81 | 2 |10 | 6 | b2 | 82 | 2 |11 | 7 | c1 | 71 | 3 |12 | 8 | c2 | 72.5 | 3 |13 | 9 | lala | 73 | 0 |14 | 10 | A | 99 | 3 |15 | 16 | test1 | 100 | 0 |16 | 17 | trigger2 | 107 | 0 |17 | 22 | trigger1 | 100 | 0 |18 +-----------+-------------+-------+---------+19 13 rows in set 
Write functions that contain cursors

The notes here are very clear , Key knowledge points have been marked in red

 1 mysql> 2 /* If the judgment function exists, delete */ 3 DROP FUNCTION IF EXISTS fun_test; 4 /* The ending operator is $*/ 5 DELIMITER $ 6 /* Create functions , Add points to each student who meets the requirements , The added score cannot exceed the given value max_score*/ 7 CREATE FUNCTION fun_test(max_score decimal(10,2)) 8 RETURNS int 9 BEGIN10 /* Define real time StudentId The variable of */11 DECLARE var_studentId int DEFAULT 0;12 /* Define the variables for the calculated fraction */13 DECLARE var_score decimal(10,2) DEFAULT 0;14 /* Define the cursor end flag variable */15 DECLARE var_done int DEFAULT FALSE;16 /* Create cursor */17 DECLARE cur_test CURSOR FOR SELECT studentid,score from students where classid<>0;18 /* At the end of the cursor var_done For true, It can be used later var_done To determine whether the cursor ends */19 DECLARE CONTINUE HANDLER FOR NOT FOUND SET var_done=TRUE;20 /* Open cursor */21 OPEN cur_test;22 /* Use Loop Loop through the cursor */23 select_loop:LOOP24 /* First get the data of the current row , Then put the data of the current row into var_studentId,var_score in , If there are no columns ,var_done Will be set to true*/25 FETCH cur_test INTO var_studentId,var_score;26 /* Through var_done To determine whether the cursor has ended , Exit loop */27 IF var_done THEN28 LEAVE select_loop;29 END IF;30 /* Yes var_score Add random value to the value , No more than the given score */31 set var_score = var_score + LEAST(ROUND(rand()*10,0),max_score);32 update students set score = var_score where studentId= var_studentId;33 END LOOP;34 /* Close cursor */35 CLOSE cur_test;36 /* Return the result : You can return the required content according to the actual situation */37 RETURN 1;38 END $39 /* The terminator is set to ;*/40 DELIMITER ;41 Query OK, 0 rows affected
Call function
1 mysql>2 /* Arguments are 8, The upper limit of bonus is 8 */3 select fun_test(8);4 +-------------+5 | fun_test(8) |6 +-------------+7 | 1 |8 +-------------+9 1 row in set
View results

Compared with the original score value , I found that the score added a random value , But not more than a given score 8

 1 mysql> select * from students; 2 +-----------+-------------+-------+---------+ 3 | studentid | studentname | score | classid | 4 +-----------+-------------+-------+---------+ 5 | 1 | brand | 105.5 | 1 | 6 | 2 | helen | 98.5 | 1 | 7 | 3 | lyn | 97 | 1 | 8 | 4 | sol | 97 | 1 | 9 | 5 | b1 | 89 | 2 |10 | 6 | b2 | 90 | 2 |11 | 7 | c1 | 76 | 3 |12 | 8 | c2 | 73.5 | 3 |13 | 9 | lala | 73 | 0 |14 | 10 | A | 100 | 3 |15 | 16 | test1 | 100 | 0 |16 | 17 | trigger2 | 107 | 0 |17 | 22 | trigger1 | 100 | 0 |18 +-----------+-------------+-------+---------+19 13 rows in set
View the trigger log

The scores that meet the conditions to be modified are 9 Information , It has been recorded in the log by trigger

 1 mysql> 2 /* The last one wrote triggers , When modifying students The log record is triggered when the table is opened */ 3 select * from triggerlog; 4 +----+--------------+---------------+-----------------------------------------+ 5 | id | trigger_time | trigger_event | memo | 6 +----+--------------+---------------+-----------------------------------------+ 7 | 1 | after | insert | new student info,id:21 | 8 | 2 | after | update | update student info,id:21 | 9 | 3 | after | update | delete student info,id:21 |10 | 4 | after | update | from:test2,101.00 to:trigger2,106.00 |11 | 5 | after | update | from:trigger2,106.00 to:trigger2,107.00 |12 | 6 | after | update | delete student info,id:11 |13 | 7 | after | update | from:brand,97.50 to:brand,105.50 |14 | 8 | after | update | from:helen,96.50 to:helen,98.50 |15 | 9 | after | update | from:lyn,96.00 to:lyn,97.00 |16 | 10 | after | update | from:sol,97.00 to:sol,97.00 |17 | 11 | after | update | from:b1,81.00 to:b1,89.00 |18 | 12 | after | update | from:b2,82.00 to:b2,90.00 |19 | 13 | after | update | from:c1,71.00 to:c1,76.00 |20 | 14 | after | update | from:c2,72.50 to:c2,73.50 |21 | 15 | after | update | from:A,99.00 to:A,100.00 |22 +----+--------------+---------------+-----------------------------------------+23 15 rows in set  
Cursor execution
Follow the example above , Analyze the execution process of this cursor .
1、 We set up a cursor , The source is from student Student table .
2、 There is an indicator in the cursor , When you open the cursor , Will execute the cursor corresponding to select Sentence , This indicator will point to select The first line in the result .
3、 When calling fetch Cursor name , Will get the data of the current row , If the current row has no data , Will trigger NOT FOUND Abnormal .
When triggered NOT FOUND When it's unusual , We can use a variable to mark , As above :DECLARE CONTINUE HANDLER FOR NOT FOUND SET var_done=TRUE;
Change the variables var_done The value of is set to TURE, You can go through the loop var_done The value of controls the exit of the loop :LEAVE select_loop;.
If the current row has data , Then save the current row data to the corresponding variable , And point the cursor index to the next line of data , The following statement :FETCH cur_test INTO var_studentId,var_score;

Summary

1、 Cursors are used to traverse query results .
2、 The use of cursors : Declaration cursor 、 Open cursor 、 Traversal cursor 、 Close cursor .
3、 Cursors are mainly used for looping 、 Storage process 、 Use in functions , Used to query the result set .
4、 The disadvantage of cursors is that they can only operate line by line , In the case of a large amount of data , Is not applicable , Too slow . Most databases are for collections , The business will be more complicated , There will be deadlock when using cursor , Affect other business operations , Not an option . When there's a lot of data , Using cursors can lead to low memory .

 

 

 

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