Question

Take tables: User, Comment, Snippet.

A User can have many Snippets.
A Snippet can have many Comments.
A User can leave many Comments.

In turn, when I diagram things, I end up with something like a triangle.

User 1-------------* Comment
      \           / 
       \         /
        *Snippet 1

Answer 1

Of course a database can have relationships like that:

Users
  id
  name
  address

Snippets
  id
  user_id
  body

Comments
  id
  body
  snippet_id
  user_id

Examples:

--Get all comments by a user
SELECT * FROM comments WHERE user_id = 1

--Get all snippets by a user
SELECT * FROM snippets WHERE user_id = 1

--Get all comments on a snippet
SELECT * FROM comments WHERE snippet_id = 1

--Get all comments on a particular snippet by a particular user
SELECT * FROM comments WHERE snippet_id = 1 AND user_id = 1