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您的局部變量與表列同名。 這樣，您就永遠不會將局部變量與列進行比較，而始終將局部變量與局部變量本身進行比較。

您的查詢需要恰好返回一行來提供 id 變量

select following_id into id from user_following
    where user_been_following_id = user_been_following_id
    and  user_following_id =  user_following_id;

user_been_following_id 和 user_following_id 在所有實例中都被解釋為局部變量，因此翻譯如下

select following_id into id from user_following
    where 1 = 1
    and  1 = 1;

其中返回 user_following 的所有行。要解決此問題，請重命名您的局部變量，例如

CREATE DEFINER=`root`@`localhost` PROCEDURE `un_follow`(
  local_user_been_following_id int,
  local_user_following_id int
)
BEGIN
    declare id int;
    select following_id into id from user_following
        where user_been_following_id = local_user_been_following_id
        and  user_following_id =  local_user_following_id; 
    
    delete from user_following 
    where following_id = id;
END

（假設表 user_following 上沒有名為 local_user_been_following_id 或 local_user_following_id 的列）

另請參閱此處： https://dev.mysql.com/doc/ refman/8.0/en/local-variable-scope.html