MySQL is intercepted using GROUP_CONCAT () function data

Encounter problems:

　　There is a demand for the project, MySQL is stored in the tree data. Now given a node, all nodes need to remove this node from Mysql database. Using MySQL function.

　　Function is as follows:

CREATE DEFINER=`root`@`%` FUNCTION `getMemberChildList`(rootId BIGINT) RETURNS mediumtext CHARSET utf8
BEGIN 
       DECLARE str VARCHAR(60000) ; 
       DECLARE cid VARCHAR(60000) ; 
       
			 SET GLOBAL group_concat_max_len=15120;
			SET SESSION group_concat_max_len=15120;
			 
       SET str = ''; 
       SET cid =cast(rootId as CHAR); 
             
       WHILE cid is not null 
			 DO 
         SET str= concat(str,',',cid); 
		 	SELECT group_concat(member_id) INTO cid from mall.ums_member_grade where FIND_IN_SET(parent_id,cid) > 0;
       END WHILE; 
       RETURN str; 
END

　  Start, less data when there is no problem, as the amount of data increases, there have been cases query results are truncated, resulting in the project being given.

solution:

　　After investigation, it was found group_concat () function has a default value.　　

　　About group_concat function:

　　1, a small amount of data when group_concat no problem, but when the amount of data to play there will be problems

　　2, group_concat: default length is connectable 1024; If you have set the maximum length, this length will be taken over to the length;

　　After 3, in the query (select) statements, using group_concat, limit will fail;

group_concat_max_len = 5120

　   Two ways to solve:

　　1 , modify the MySQL configuration file:

# Length to be set 
group_concat_max_len = 5120

　　2, can also be used to set sql statement:

SET GLOBAL group_concat_max_len=5120;
SET SESSION group_concat_max_len=5120;

At last:

　　Fundamentals still not so strong, it was just a lot of functions in the understanding stage. We should pay close attention to the basics of catching up. Or else having problems can only be a time of trial and error, or ask someone else.

 

Reference blog:

https://www.cnblogs.com/Steven-shi/p/7106495.html?utm_source=itdadao&utm_medium=referral。