database model for a rotating rounds system - mysql

Question

Struggling with how to correctly model a database for a fairly simple app i'm putting together. App is just a simple thing i'm using to learn a few framework. Each week someone in the office has to take a turn bringing in beers. This is done on a rotation, we take it in turns following a specific order. Occasionally someone isn't around and they skip their turn but will have to take it the following week (or indeed the first subsequent week they can). Obviously we also have staff come and staff leave so this needs to be accounted for. Initially i'd modelled this in terms of drinkers and rounds. Each time someone bought a round they would be entered into the rounds table, whoever is first in the order with the least rounds is up next. This works perfectly until someone new joins as without inserting dummy data to cover all their "missing" rounds it will be their turn every time until they have caught up on the number of rounds everyone else has done.
This is currently what I have:
CREATE TABLE `rounds` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`drinker_id` int(11) NOT NULL,
`description` text COLLATE utf8_unicode_ci NOT NULL,
`created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
PRIMARY KEY (`id`)
)
CREATE TABLE `drinkers` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`email` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
)
Any ideas how I can re-model this in a way that will allow for people coming, and going, and people skipping their turn?
Thanks.

Answer 1

You only need one table like
CREATE TABLE `drinkers` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`email` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`is_available` TINYINT(1) NOT NULL DEFAULT 0,
`last_buy` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
PRIMARY KEY (`id`),
INDEX(`last_buy`)
)
and you can get the person who needs to buy drinks like this
SELECT * FROM drinkers
WHERE is_available
ORDER BY last_buy ASC
LIMIT 1

Answer 2

I would keep the drinkers (but add a flag if someone is sick - 0/1) and put rounds as this
CREATE TABLE `rounds` (
`drinker_id` int(11) NOT NULL,
`passed` int(11) NOT NULL,
`passed_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
UNIQUE KEY (`id`)
)
Then put in round a record for every user with his ID and 0 for passed, 0000-00-00 00:00:00 for passed at. When someone buys drinks mark that in the 'rounds' table, with making passed to passed + 1 and updating the date. If some of the drinkers leaves work, remove him from rounds. If a new one comes add him to rounds with passed equals to the max passed values (give them a fresh start :) ). If some one is sick, he obviously can not buy drinks so someone else should. To define who should buy drinks, run this
SELECT r.passed, r.drinker_id
FROM drinkers d
JOIN rounds r ON d.id = r.drinker_id
WHERE d.sick = 0
ORDER BY r.passed DESC, d.id ASC
This will give you the driker with least times buying a drink.
In rounds table you will have somethin like that
drinker id, passed, date_passed
====================================
1 0 0000-00-00
2 1 2013-03-01
3 0 0000-00-00