深刻理解MySQL8游标处理中not found

深刻理解MySQL8游标处理中not found

最近使用MySQL的游标,在fetch循环过程中,程序总是提前退出 ,百思不得其解,经过测试,原来是对于游标处理中not found的定义理解有误,默认是视同Oracle的游标not found定义,结果思考测试了两天,终于走出了思维定式。

1. 问题描述

MySQL版本,8.0.16 。

存储过程如下:

复制代码
CREATE DEFINER=`root`@`%` PROCEDURE `pro_test_nofound_cursor`()
begin
  
  declare v_done int default 1 ;
  declare v_name varchar(10);
  declare v_date date;
	declare v_string text;
	declare v_for_nofound varchar(10);
	declare v_counter int default 0;
	
  declare cur_stud1 cursor for select t.name ,t.birthday from tb_student t where t.grade >= 70 and t.grade < 80 order by t.grade desc limit 3;
  declare continue handler for not found set v_done = 0;

  #使用游标前打开游标
  open cur_stud1 ;
	set v_string = '';
  
	cur_loop: loop
		fetch next from cur_stud1 into v_name ,v_date;
		set v_counter = v_counter + 1;
		if v_done = 0 then 
			leave cur_loop;
		end if;
	 
	 -- 此查询无结果,是空。	
	 select t.name  into v_for_nofound 	from tb_student t where t.grade >= 101  order by t.grade desc limit 1;
	 
	 set v_string = concat(v_string,' stud1:',v_name , ' :',v_date);

	end loop cur_loop;

	close cur_stud1 ;
	
	select v_string;
	select v_counter;
	
end

游标记录是3条记录,但是查询结果,只反馈一条记录值。

游标理解应该循环3次!!!,但是只返回了一条记录。
为什么 ???

结果如下:

复制代码
mysql> call pro_test_nofound_cursor();
+-------------------------------+
| v_string                      |
+-------------------------------+
|  stud1:CJXBCEXCOF :2023-09-18 |
+-------------------------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

mysql> call pro_test_nofound_cursor();
+-------------------------------+
| v_string                      |
+-------------------------------+
|  stud1:CJXBCEXCOF :2023-09-18 |
+-------------------------------+
1 row in set (0.00 sec)

+-----------+
| v_counter |
+-----------+
|         2 |
+-----------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

结果说明:

记录返回:只有1条

计数器:是2

2. 问题分析

MySQL文档:
MySQL定义not found的说明

NOT FOUND: Shorthand for the class of SQLSTATE values that begin with '02'. This is relevant within the context of cursors and is used to control what happens when a cursor reaches the end of a data set. If no more rows are available, a No Data condition occurs with SQLSTATE value '02000'. To detect this condition, you can set up a handler for it or for a NOT FOUND condition.

复制代码
DECLARE CONTINUE HANDLER FOR NOT FOUND   BEGIN
     -- body of handler   
 END; 

For another example, see Section 13.6.6, "Cursors". The NOT FOUND condition also occurs for SELECT ... INTO var_list statements that retrieve no rows.

说明:

SQLSTATE value '02000' 和 NOT FOUND 是等价的,那么NOT FOUND 就不是cursor所专属的状态值。因此在循环中,如果出现了查询没有结果的情况,那么将直接 触发v_done = 0 ,并非cursor的fetch 触发的结果。

注意:与Oracle游标访问的notfound状态值是不同的,oracle是专用于cursor,而MySQL是notfound状态是所有SQL共用的!!!

惯性思维,困扰了两天。

declare continue handler for not found set v_done = 0;

3. 问题解决

在游标循环中最后增加一行,强制设置为1 ;

set v_done = 1;

程序只有在fetch的时候,产生的v_done状态,才能触发退出循环。

修改后的程序如下:

复制代码
CREATE DEFINER=`root`@`%` PROCEDURE `pro_test_nofound_cursor`()
begin
  
  declare v_done int default 1 ;
  declare v_name varchar(10);
  declare v_date date;
	declare v_string text;
	declare v_for_nofound varchar(10);
	declare v_counter int default 0;
	
  declare cur_stud1 cursor for select t.name ,t.birthday from tb_student t where t.grade >= 70 and t.grade < 80 order by t.grade desc limit 3;
  declare continue handler for not found set v_done = 0;

  #使用游标前打开游标
  open cur_stud1 ;
	set v_string = '';
  
	cur_loop: loop
		fetch next from cur_stud1 into v_name ,v_date;
		set v_counter = v_counter + 1;
		if v_done = 0 then 
			leave cur_loop;
		end if;
	 
	 -- 此查询无结果,是空。	
	 select t.name  into v_for_nofound 	from tb_student t where t.grade >= 101  order by t.grade desc limit 1;
	 
	 set v_string = concat(v_string,' stud1:',v_name , ' :',v_date);
	 set v_done = 1;
	 
	end loop cur_loop;

	close cur_stud1 ;
	
	select v_string;
	select v_counter;
	
end

执行结果:

复制代码
mysql> call pro_test_nofound_cursor();
+-----------------------------------------------------------------------------------------+
| v_string                                                                                |
+-----------------------------------------------------------------------------------------+
|  stud1:CJXBCEXCOF :2023-09-18 stud1:FIDLSJAYFS :2023-11-08 stud1:KEVQMOCIEW :2023-09-06 |
+-----------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

+-----------+
| v_counter |
+-----------+
|         4 |
+-----------+
1 row in set (0.01 sec)

Query OK, 0 rows affected (0.01 sec)

执行结果正确,返回了3条记录,计数器值是4 。

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