我想用while循环编写一个嵌套游标，这样当表\u a中的列\u name与表\u b的col1匹配时，就可以从表\u b中获得结果。
如果从表\u b中选择col1、col2、col3，其中col1=tbla\u col1为空，则转到下一行并返回col1。
我需要将列结果保存在变量中。

Delimiter $$
  DROP PROCEDURE IF EXISTS sp_test;
  CREATE PROCEDURE sp_test() 
  begin 
    DECLARE done, done1 int DEFAULT 0; 
    DECLARE tblA_col1 varchar(255); 
    DECLARE tblB_col1 varchar(255);
    DECLARE tblB_col2 varchar(255);
    DECLARE tblB_col3 varchar(255);
      DECLARE curA CURSOR FOR   SELECT column_name   FROM   Table_A; 
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1; 
    open curA; 

      WHILE (done = 0) do
      FETCH next   FROM  curA   INTO  tblA_col1;
     DECLARE curB CURSOR FOR   select col1, col2, col3 from Table_B where col1 = tblA_col1;
      DECLARE CONTINUE HANDLER FOR NOT FOUND SET done1 = 1; 
      open curB;
    Fetch next from curB into tblB_col1,tblB_col2,tblB_col3;
    select tblB_col1;
   select tblB_col2;
    select tblB_col3;
    close curB;
  if tblB_col1 is null then 
  tblB_col1 = tblB_col1;
  end if;

    end while;
    close curA;
    end;
    $$
  --  call sp_test

我是新来的游标和我有问题，而使用嵌套的路缘。你能帮忙吗？