question on how to set up a table

[tubedogg]

Which of the following three would likely be least intensive on a busy system?

CREATE TABLE forum (
    userid
    forumid
) 

Notes: possibly up to 500 or 600 forumids. Would have one row per forumid per user (possibly up to 50,000 or more users). Not necessarily all userid's will have a row for each forumid.

CREATE TABLE forum (
    userid,
    forum1,
    forum2,
    forum3,
    ...,
    ...,
    ...,
    forum500,
    forum501,
    forum502
) 

Notes: One column for each forum, up to 500 or 600. One row per user. Not all columns will necessarily have a value for every user.

ALTER TABLE user ADD COLUMN forumids MEDIUM TEXT; 

Notes: Store forumids as comma-delimited list, explode'd into an array for use in the program. Length of actual list varies per user, but could be up to 500 or 600 items per list.

Any thoughts on how I should go about this?
I have obviously left off the details of each column such as type etc. but this is the basic idea.

[kcabobert]

My personal thoughts would be that the last would be the best in this case (for what I imagine your trying to do.) I think having a table with 600 columns and 50,000 rows would be a big mess (my thoughts.) The first table could potentially have 30,000,000 rows in a two-column table, which could also be an enormous fiasco. I think that the middle one gives the best all around solution, exploding the forumids into an array with a for loop (or while) would make that a quick easy solution.