mysql - 简单的选择过程

Question

我在 MySQL 中有这个过程

DROP PROCEDURE IF EXISTS `AddNotificationOnPosts`;

DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `AddNotificationOnPosts`(from_user INT(11),on_post_id INT(11),in_group_id INT(11))
BEGIN
    DECLARE num_rows INT DEFAULT 0;

    SELECT count(notification_id) FROM notifications_posts 
    WHERE from_user = 1;

END $$
DELIMITER ;

还有这张桌子

notification_id from_user   on_post_id  in_group_id date
2   1   162 3   2012-07-11 12:01:08
3   19  163 1   2012-07-11 12:03:26
4   19  164 1   2012-08-10 17:42:36
5   1   165 3   2012-08-29 12:14:01
6   1   165 3   2012-08-29 12:14:29

当我执行：

SET @p0 =  '1';
SET @p1 =  '2';
SET @p2 =  '3';
CALL `AddNotificationOnPosts` ( @p0 , @p1 , @p2 );

它给了我：

count(notification_id)
5

为什么？因为它们只是 user_id 1 的 3 条记录

还有我如何使这个程序：

1. 未设置值时给出错误或警告

2. 从过程中获取变量，例如

   SELECT count(notification_id) FROM notifications_posts 
   WHERE from_user = @from_user;
   

Answer 1

您应该为参数使用不同的名称：

CREATE PROCEDURE AddNotificationOnPosts
(arg_from_user INT(11),arg_on_post_id INT(11),arg_in_group_id INT(11))
BEGIN
    DECLARE num_rows INT DEFAULT 0;

    IF(arg_from_user  IS NULL OR arg_from_user = '')
    THEN 
        SELECT "Error:Invalid argument for userID" AS error_code;
    ELSE
        SELECT count(notification_id) 
        INTO num_rows
        FROM notifications_posts 
        WHERE from_user = arg_from_user;
    END IF;

    SELECT num_rows;
END $$