Detailed analysis on the use of MySQL stored procedure loop

Catalog

brief introduction

Scene description

Solution

Case study

summary

brief introduction

The syntax of every database language is basically similar , But for their own characteristics （ function 、 Stored procedure, etc ） The usage of is not the same , like Oracle And Mysql Stored procedures are written in many different ways , Here is mainly to share with you MySql The method of using cursor loop in stored procedure .

Scene description

Let's take a simple scenario , First of all, we may have such a situation , Test results (t_achievement) There's a pile of sql Script processing , Need to rely on another student table (t_student) The data is used to summarize the test scores of some students and record them in the score summary table （t_achievement_report）.

Solution

One way is to get the student table data to be summarized through code priority , Then the student information data will be transferred to the score summary business code for processing one by one according to the score summary process .

Another way is also our topic today , That is to do it through stored procedures .

Case study

Create table statement ：

-- Student information sheet DROP TABLE IF EXISTS t_student;CREATE TABLE `t_student` ( `id` BIGINT(12) NOT NULL AUTO_INCREMENT COMMENT ' Primary key ', `code` VARCHAR(10) NOT NULL COMMENT ' Student number ', `name` VARCHAR(20) NOT NULL COMMENT ' full name ', `age` INT(2) NOT NULL COMMENT ' Age ', `gender` CHAR(1) NOT NULL COMMENT ' Gender （M： male ,F： Woman ）', PRIMARY KEY (`id`), UNIQUE KEY UK_STUDENT (`code`)) CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;-- Student transcript DROP TABLE IF EXISTS t_achievement;CREATE TABLE `t_achievement` ( `id` BIGINT(12) NOT NULL AUTO_INCREMENT COMMENT ' Primary key ', `year` INT(4) NOT NULL COMMENT ' school year ', `subject` CHAR(2) NOT NULL COMMENT ' subject （01： Chinese language and literature ,02： mathematics ,03： English ）', `score` INT(3) NOT NULL COMMENT ' score ', `student_id` BIGINT(12) NOT NULL COMMENT ' Student id', PRIMARY KEY (`id`) ) CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;-- Summary of grades DROP TABLE IF EXISTS t_achievement_report;CREATE TABLE `t_achievement_report` ( `id` BIGINT(12) NOT NULL AUTO_INCREMENT COMMENT ' Primary key ', `student_id` BIGINT(12) NOT NULL COMMENT ' Student id', `year` INT(4) NOT NULL COMMENT ' school year ', `total_score` INT(4) NOT NULL COMMENT ' Total score ', `avg_score` DECIMAL(4,2) NOT NULL COMMENT ' average ', PRIMARY KEY (`id`) ) CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

Initialization data ：

INSERT INTO t_student(id, CODE, NAME, age, gender) VALUES(1, '2022010101', ' Xiao Zhang ', 18, 'M'),(2, '2022010102', ' petty thief ', 18, 'F'),(3, '2022010103', ' Xiao Ming ', 18, 'M');INSERT INTO t_achievement(YEAR, SUBJECT, score, student_id) VALUES(2022, '01', 80, 1),(2022, '02', 85, 1),(2022, '03', 90, 1),(2022, '01', 60, 2),(2022, '02', 90, 2),(2022, '03', 98, 2),(2022, '01', 75, 3),(2022, '02', 100, 3),(2022, '03', 85, 3);

stored procedure ：

Take the above scenario as an example , Use stored procedure loops to process data . Write a stored procedure , Summarize the above data and the scores of each student .

-- If the stored procedure exists , Delete the stored procedure first DROP PROCEDURE IF EXISTS statistics_achievement;DELIMITER $$-- Define stored procedures CREATE PROCEDURE statistics_achievement()BEGIN -- Define whether the variable record cycle processing is completed DECLARE done BOOLEAN DEFAULT FALSE; -- Define variables to pass to students idDECLARE studentid BIGINT(12);-- Define cursors DECLARE cursor_student CURSOR FOR SELECT id FROM t_student;-- Definition CONTINUE HANDLER, When the loop ends done=trueDECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done=TRUE;-- Open cursor OPEN cursor_student;-- Iterate through REPEAT -- One read at a time cursor FETCH cursor_student INTO studentid; -- Calculate the total score 、 The average score is inserted into the summary table INSERT INTO t_achievement_report(student_id, `YEAR`, total_score, avg_score)SELECT studentid, `YEAR`, SUM(score), ROUND(SUM(score) / 3, 2) FROM t_achievement t1 WHERE student_id = studentid AND NOT EXISTS(SELECT 1 FROM t_achievement_report t2 WHERE student_id = studentid AND t1.year = t2.year) GROUP BY `YEAR`;-- End of cycle , It means to wait until done=true when , End of cycle REPEATUNTIL done END REPEAT;-- Query results , Only the last one found will be shown SELECT studentid;-- Close cursor CLOSE cursor_student;END$$DELIMITER ;-- Execute stored procedures CALL statistics_achievement();

Execution results , Return query results 3, The last student record id

summary

Stored procedures also have powerful functions , If it's a DBA So writing stored procedures is a matter of minutes , But as a yard farmer who specializes in business, I still It is not recommended to use stored procedures to write business code . Former company colleagues adapted to writing stored procedures , If there are business changes, you can directly use stored procedures from time to time , In the end, it's just a lot of stored procedure code , A stored procedure has hundreds or thousands of lines sql I feel dizzy when I see the wharf , If something goes wrong, it is very difficult to maintain , People who are a little unfamiliar dare not act rashly , Today, I just give you a chestnut to explain the loop in the stored procedure. Please don't mind .

In short, I think stored procedures are mainly used to temporarily process some data for convenience , In particular, some businesses have been transformed greatly , If you need to cut data, you can't write business code one by one .

This is about MySql This is the end of the detailed analysis of the use of stored procedure loops , More about MySql For the content of stored procedure loop, please search the previous articles of SDN or continue to browse the related articles below. I hope you can support SDN more in the future ！