MySQL Procedure 变量在 count(*) 上接收 null

为什么当我做Select count(*) From table1时，我得到300，但如果我做SELECT end = COUNT(*) FROM table1;，则返回NULL

以下是小提琴的https://dbfiddle.uk/ZHzoaztV个例子

代码片段:

CREATE TABLE table1(
  start int NOT NULL,
  id int PRIMARY KEY AUTO_INCREMENT,
  counter int NOT NULL,
  difference int NOT NULL,
  end int NOT NULL
);

CREATE PROCEDURE doWhile()
BEGIN
  DECLARE i INT DEFAULT 1;
  DECLARE start INT DEFAULT 120;
  DECLARE counter INT DEFAULT 1;
  DECLARE end INT DEFAULT 300;
WHILE (i <= end) DO
  INSERT INTO table1 VALUES (start,null,counter,start+counter,end); 
  SET i = i+1;
  SET counter = counter+1;
END WHILE;
END;
CALL doWhile();
SELECT * FROM table1;


CREATE PROCEDURE insertMore()
BEGIN
  DECLARE start INT;
  DECLARE counter INT DEFAULT 1;
  DECLARE end INT;

  SELECT end = COUNT(*) FROM table1;
  SELECT start = MAX(id)+1 FROM table1;
  -- SELECT COUNT(*) FROM table1;

  WHILE (counter <= end) DO
    INSERT INTO table1 VALUES (start,null,counter,start+counter,end); 
    SET counter = counter+1;
  END WHILE;
END;
CALL insertMore();
SELECT * FROM table1;

我希望返回300，所以希望我的函数能做对