back_mysql.py is creating sql like:

select distinct(_issue.id),rhs0_._order from 
_issue,issue_topic LEFT OUTER JOIN _priority as rhs0_ 
on _issue._priority=rhs0_.id where 
_issue.id=issue_top...

mixing the two join syntaxes isn't valid.

i hacked it by changing line 768 to:

        frum = ' join '.join(frum)

cheesy because there's no "on" so it does a cartesian 
join, but it works.