MySQL8 嵌套游标循环遍历

MySQL8 嵌套游标循环遍历

1. 游标的特点

官网介绍：

MySQL supports cursors inside stored programs. The syntax is as in embedded SQL. Cursors have these properties:

• Asensitive: The server may or may not make a copy of its result table
• Read only: Not updatable
• Nonscrollable: Can be traversed only in one direction and cannot skip
  rows

Cursor declarations must appear before handler declarations and after variable and condition declarations.

特性：
不敏感：数据库服务器可以，或不可 复制其结果表 ，如何理解，疑惑？？
只读：不能更新数据
不可滚动：游标只能单方向访问，不能跳行
定义的时候要注意：
游标声明必须在其他变量之后，handler 的声明在游标定义之后。

2. 嵌套游标遍历

重点是对于handler控制的标志变量进行赋值，保证嵌套的游标能够正常遍历完成。关键赋值：

		#cur_stud2完成后，v_done=0 ,重置游标标志
		set v_done = 1;

程序如下

CREATE DEFINER=`root`@`%` PROCEDURE `pro_loop_cursor`()
begin
  
  declare v_done int default 1 ;
  declare v_name varchar(10);
  declare v_grade int ;
  declare v_string text;
	declare v_counter int default 0;
	
  declare cur_stud1 cursor for select t.name ,t.grade from tb_student t where t.grade >= 70 and t.grade < 80 order by t.grade desc limit 3;
	declare cur_stud2 cursor for select t.name ,t.grade from tb_student t where t.grade >= 90 and t.grade < 100 order by t.grade desc limit 2;
  #游标结标志，必须在游标声明后声明！！！否则报错
  declare continue handler for not found set v_done = 0;

	set v_string = '';  
	
	#外循环，打开游标
	open cur_stud1 ;
	
	cur_loop1:loop
			#内循环，打开游标
			open cur_stud2 ;
			fetch next from cur_stud1 into v_name ,v_grade;
			#外循环，游标标志判断
			if v_done = 0 then 
				leave cur_loop1;
			end if;
			
			# 外循环游标数据遍历
			set v_string = concat(v_string,' stud1:',v_name , ' :',v_grade);
			
			# 内循环开始
			cur_loop2: loop
				fetch next from cur_stud2 into v_name ,v_grade;
				#内循环，游标标志判断
				if v_done = 0 then 
					leave cur_loop2;
				end if;
				# 内循环游标数据遍历
				set v_string = concat(v_string,' stud2:',v_name , ' :',v_grade);
								
			end loop cur_loop2; # 内循环结束
			#关闭内循环游标
			close cur_stud2 ;
			
			#计数器 +1
			set v_counter = v_counter + 1;
			# cur_stud2完成后，v_done=0 ,重置游标标志
			set v_done = 1;
			
	end loop cur_loop1; # 外循环结束
	#关闭外循环游标
	close cur_stud1 ;
	
	select v_counter ;
	select v_string;
	
end

执行结果：
外循环记录是3条，70-80分的前三名，内循环记录是2条，90-100分的前两名，需要循环3次。
v_counter 是3 。
每个stud1对应两条stud2记录。

mysql> call pro_loop_cursor();
+-----------+
| v_counter |
+-----------+
|         3 |
+-----------+
1 row in set (0.01 sec)

+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| v_string                                                                                                                                                                                      |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|  stud1:CJXBCEXCOF :79 stud2:WKJVXXFKGY :99 stud2:PYRESDIMBG :99 stud1:FIDLSJAYFS :79 stud2:WKJVXXFKGY :99 stud2:PYRESDIMBG :99 stud1:KEVQMOCIEW :78 stud2:WKJVXXFKGY :99 stud2:PYRESDIMBG :99 |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

Query OK, 0 rows affected (0.01 sec)