Question

Task:

At present, the database knows two types of messages:

1. Messages that a user posts and that are public for anyone and everyone to read
2. Messages that a user posts and that are non-public.

These messages can only be read by users that the posting user has marked as friends.
In this step, you should add a third type of message. This third type of message should be readable by specified recipients only.

This means the database needs to provide the following:

• A way of distinguishing between the three types of messages. This involves a change to the Message table.
• A way of specifying who the recipients of a particular message are. This will probably require an additional table.

Your job is to implement the necessary changes and additional table for this purpose and any keys and foreign key
relationships required.

here are two existing tables witch relate to the task(copies from my db).

User table

CREATE TABLE IF NOT EXISTS `User` (
  `user_id` int(10) unsigned NOT NULL auto_increment,
  `given_name` varchar(60) default NULL,
  `surname` varchar(60) default NULL,
  `address` varchar(255) default NULL,
  `city_id` int(10) unsigned NOT NULL,
  `date_of_birth` datetime default NULL,
  `email` varchar(80) default NULL,
  PRIMARY KEY  (`user_id`),
  KEY `ix_user_surname` (`surname`),
  KEY `ix_user_given_name` (`given_name`),
  KEY `ix_user_name` (`given_name`,`surname`),
  KEY `ix_user_date_of_birth` (`date_of_birth`),
  KEY `ix_user_email` (`email`),
  KEY `ix_user_city_id` (`city_id`)
) ENGINE=InnoDB

Message table

CREATE TABLE IF NOT EXISTS `Message` (
  `message_id` int(10) unsigned NOT NULL auto_increment,
  `owner_id` int(10) unsigned default NULL,
  `subject` varchar(255) default NULL,
  `body` text,
  `posted` datetime default NULL,
  `is_public` tinyint(4) default '0',
  PRIMARY KEY  (`message_id`),
  KEY `ix_message_owner_id` (`owner_id`)
) ENGINE=InnoDB

Answer 1

Ok, so is_public give you the ability to distinguish between two types (e.g. is_public = ‘0’ means private, and is_public = ‘1’ means public). But now you have a new concept of specified receipts, so the yes/no model won’t work anymore b/c you have 3 types. Usually in this situation you can switch to a flag or type column.

So maybe make a message_type column that is one of ‘PUBLIC’, ‘PRIVATE’, ‘SPECIFIED’ or something like that.

After that it sounds like you need at least two more tables. Users must be able to specify friends and users must be able to specify users to receive particular messages.