What's the basis to choose SQL over HQL in hibernate for some queries?

I am using hibernate in my project and I have always used HQL. However, I have seen in other projects where at times SQL queries are used by createSqlQueries, rather than HQL. I wanted to know, what could be the deciding factor to choose SQL over HQL in certain scenarios. Also if there are some queries which can't be performed by HQL and we need to choose only SQL, please cite example.

via Chebli Mohamed