Question

Possibly a difficult problem to solve… Also I realize there may be more than one right answer here, but the main thing is really about how to automate the process.

I’ve got lots of pupils in a InnoDb table. In another table I’ve got their favourite choices for roommates.

• Most have chosen two or three favourite roommates.
• A few have chosen none.
• Some are chosen by up to four others.
• Some are chosen by none.
• Most pupils are chosen by one or two other pupil(s).

In the thirds table I have the rooms that are going to be filled. The rooms have from two to three beds. There aren’t any one-bed or four-bed rooms.

As far as constraints go, it all works. The problem is, how do I automate this process with MySQL (or other programming, preferably PHP) so that I make everybody happy?

I’ve made the assumption that the first roommate chosen is the one the pupil wants most, thus I’ve rated the choises from one to four based on the order of mention. Of course, sometimes this crash and two pupils have the same choice on number one. And then there is the problem of the one’s that aren’t chosen by anybody, but you can assume that they will be mature about it.

Answer 1

Well for starters, I would save a backup of the database, just in case your procedure making goes awry:

mysql -h host -u username -p password database_name > database_name_backup.sql

You’re going to need a join table for the rooms and pupils as well, if you don’t already have one. So something like:

CREATE TABLE room_pupil (
  room_id int NOT NULL,
  pupil_id int NOT NULL
);

Note that, while you did a good job describing your tables in your question, providing your exact DDL for your table definitions (the statements that replicate the table and relationships) helps people give answers to these types of questions. Just for future reference, nothing hurt here 🙂

From there, you can run the following type of script to create a stored procedure you could then call to arrive at a good solution

(Note: This is not actually going work for you! You will need to fix this up to make the matching better, and tailor it to your specific data types! Might even be a syntax error or two in this, I’m writing it a little blind. Learning to fish is better than getting a fish, though ^^)

DELIMITER $$ 
DROP PROCEDURE IF EXISTS RoommateMatching$$ 
CREATE PROCEDURE RoommateMatching() 
BEGIN 
  DECLARE no_more_pupils int DEFAULT 0;
  DECLARE current_pupil CURSOR FOR 
SELECT pupil_id FROM pupils; 
  DECLARE CONTINUE HANDLER FOR NOT FOUND 
SET no_more_pupils = 1; 

/* create a temp table to hold the proposals */ 
CREATE TABLE temp_matches ( 
  proposing_pupil_id int NOT NULL,
  proposed_pupil_id int NOT NULL 
); 
OPEN current_pupil; 

FETCH current_pupil INTO this_pupil; 

REPEAT 

/* Coalesce -1 in so its easy to tell whether they had choices picked or not */
SELECT COALESCE(pupil_choice_id, -1) INTO next_favorite_roommate_id 
  FROM roommate_preference /* TODO Your second table, don't know its name */ 
  WHERE pupil_id = this_pupil; /* TODO since there may be more than one match here or none, might need another iterator */

SELECT COALESCE(proposed_pupil_id, -1) INTO existing_proposal 
  FROM temp_matches
  WHERE proposed_pupil_id = next_favorite_roommate_id;

IF existing_proposal < 0 THEN 
  INSERT INTO temp_matches(proposing_pupil_id, proposed_pupil_id) 
  VALUES (this_pupil, next_favorite_roommate_id); 
END IF; 
IF existing_proposal > 0 THEN
  /* TODO see if there's a better match for existing_proposal here, you write it! */
END IF;
FETCH current_pupil INTO this_pupil; 
UNTIL no_more_pupils = 1 

END REPEAT; 

CLOSE current_pupil; 
/* start an iterator on the matches and insert records into your room_pupil table! */
/* TODO you write that code! */
DROP TABLE temp_matches; 
END$$ 
DELIMITER; 

That should get you on the right track at least! Let me know if anything needs more clarification, but like I said I’m not going to write whole algorithm exactly for you, use wikipedia and the mysql man pages to learn your way into Stored Procedure stardom! 😉

source of stored procedure skeleton: http://forums.mysql.com/read.php?98,358569,358569