How can I make a MySQL SUM query return zero instead of null if there are no records?

This should do the trick:

SELECT COALESCE(SUM(rating),0) AS this_week FROM table_name 
  WHERE UNIX_TIMESTAMP(created_at) >= UNIX_TIMESTAMP() - 604800)

COALESCE is a function that will return the first non NULL value from the list.

Can't you use IFNULL(SUM(rating), 0)?