本文将举例演示游标的嵌套用法，首先建造表结构，如下：

drop table if exists `tb_user`;
create table tb_user (
	id bigint(22) not null auto_increment primary key,
    name varchar(20) not NULL,
    age  tinyint(3),
    gmt_create date,
    gmt_modified date
);

drop table if exists `tb_user_param`;
create table tb_user_param (
	id bigint(22) not null auto_increment primary key,
    user_id bigint(22) not null,
    code varchar(100) not null,
    value varchar(1000),
    gmt_create DATE,
    gmt_modified date
);

drop procedure if exists `sp_init_data`;
create procedure `sp_init_data`()
begin
	declare v_i bigint(22) default 0;
    declare v_uid bigint(22);
    declare v_nick varchar(20) default 'lanbo_';
    declare v_code_1 varchar(20) default 'address';
    declare v_code_2 varchar(20) default 'phone';
    declare v_code_3 varchar(20) default 'wangwang_id';
    declare v_value_1 varchar(20) default 'HZ.XiHu';
    declare v_value_2 varchar(20) default '1875757198';
    declare v_value_3 varchar(20) default 'shansun_';
	
    while v_i < 10 do
    	set v_i = v_i + 1;
    	insert into tb_user values (null, concat(v_nick, v_i), 23, now(), now()); 
        select LAST_INSERT_ID() into v_uid; 
        insert into tb_user_param values(null, v_uid, v_code_1, v_value_1, now(), now());
        insert into tb_user_param values(null, v_uid, v_code_2, concat(v_value_2, v_i), now(), now());
        insert into tb_user_param values(null, v_uid, v_code_3, concat(v_value_3, v_i), now(), now());
    end while;    
    commit;
end;
call sp_init_data();


drop table if exists `tb_key_value`;
create table `tb_key_value`(
	uid bigint(22) not null,
	k varchar(100),
    v varchar(100)
);

我们插入了10条数据到tb_user中，如果tb_user中未定义的字段如address，则放置在tb_user_param中，该表是个key_value的结构，由uid+code定位。

后面我们要做的是，将根据tb_user中的uid找到tb_user_param中相关记录后，将key_value信息转移到tb_key_value表中，为达到演示效果，我们使用嵌套游标操作数据，代码如下：

drop procedure if exists `sp_nested_cursor`;
create procedure `sp_nested_cursor`()
begin
	declare v_uid bigint(22);
    declare v_code varchar(100);
    declare v_value varchar(100);
    declare _done TINYINT(1) default 0;
	declare cur_user cursor for select id from `tb_user`;
    declare continue handler for not found set _done = 1;
    
    open cur_user;
    loop_xxx:loop
    	fetch cur_user into v_uid;
        if _done=1 then
        	leave loop_xxx;
        end if;
        begin
        	declare _inner tinyint(1) default 0;
        	declare cur_param cursor for select code, value 
                                         from `tb_user_param` 
                                         where user_id=v_uid;
            declare continue handler for not found set _inner = 1; 
            open cur_param;
            loop_yyy:loop
            	fetch cur_param into v_code, v_value;
                if _inner=1 then
                	leave loop_yyy;
                end if;
                insert into tb_key_value values (v_uid, v_code, v_value);
            end loop;
            commit;
        end;
    end loop;
end;
call `sp_nested_cursor`();