Writing Comments table in database

Question

I'm working on a social networking system that will have comments coming from several different locations. One could be friends, one could be events, one could be groups--much like Facebook. What I'm wondering is, from a practical standpoint, what would be the simplest way to write a comments table? Should I do it all in one table and allow foreign keys to all sorts of different tables, or should each distinct table have its own comment table? Thanks for the help!

Answer 1

A single comments table is the more elegant design, I think. Rather than multiple FKs though, consider an intermediate table - CommentedItem. So Friend, Event, Group, etc all have FKs to CommentedItem, and you create a CommentedItem row for each new row in each of those tables. Now Comments only needs one FK, to CommentedItem. For example, to get all Comments for a given Friend:

SELECT * FROM Comment c
JOIN CommentedItem ci on c.CommentedItemId = ci.CommentedItemId
JOIN Friend f on f.CommentedItemId = ci.CommentedItemId
WHERE f.FriendId = @FriendId

Answer 2

I've done both and the answer depends on the situation. For what you are trying to do, I would do a SINGLE "Comments" table, and then seperate "linker" tables. This will give you the best performance as you can achieve the "Perfect Index".

I would also recommend putting a "CommentTypeID" field in the Comments table to give a 'clue' as to which linker table you will pull from for the aditional detail.

EDIT: The CommentTypeID field should not be used in the indexes, but rather it's only for use in code.