Fix on interview 14 & 15

[wctjerry]

• Add a tmp table with two-way binding for the use of constructing a list of (user_id, user_id) base pairs by CROSS JOIN, so that it will contain all possible user_ids that may be missed in the previous approach. Check edge case: alice and sonny
• Remove a semicolon in db.sql that blocks loading the last row in the dataset
• Fix a link to Interview 15 problem set