如何在MySQL语句中使用嵌套循环和变量

我试图编写一个sql语句,在给定几个设置变量的情况下插入数据。我不想创建一个存储过程,因为除了作为管理员之外,我在任何地方都不会处理它,我也不想让这个存储过程一直存在。我只想要一份有效的声明。以下是我目前掌握的情况:

SET @app = 7;
SET @time = UNIX_TIMESTAMP(NOW());
SET @maxValue = 100;
SET @decrement = 10;
SET @category = 1;
SET @minSubcategory = 0;
SET @maxSubcategory = 19;
SET @subcategory = @minSubcategory;
subcat_loop: LOOP
  SET @idx = 0;
  insert_loop: LOOP
    SET @id = CONCAT('TempId', @idx+1);
    SELECT @name:=username FROM user WHERE id = @id;
    SET @value = @maxValue - @decrement * @idx;
    INSERT INTO data (userId, username, app, category, subcategory, value, date) VALUES
      (@id,  @name, @app, @category, @subcategory, @value, @time);
    SET @idx = @idx+ 1;
    IF @idx > 10 THEN
      LEAVE insert_loop;
    END IF;
  END LOOP insert_loop;
  SET @subcategory = @subcategory + 1;
  IF @subcategory > @maxSubcategory THEN
    LEAVE subcat_loop;
  END IF;
END LOOP subcat_loop;

但它不喜欢 SET @idx = 0 因为某种原因在循环中。我做错什么了?