Reasons for product joins

1. Stale or no stats causing optimizer to use product join
2. Improper usage of aliases in the query.
3. missing where clause ( or Cartesian product join  1=1 )
4. non equality conditions like > ,< , between   example ( date)
5. few join conditions
6.  when or conditions are used.

last but not the least   product joins are not bad always!! sometimes PJ are better compared to other types of joins