def combine_expression(self, connector, sub_expressions):
        conn = ' %s ' % connector
        return conn.join(sub_expressions)
Combine a list of subexpressions into a single expression, using
dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/Django-1.6.10/django/db/backends/__init__.py/BaseDatabaseOperations.combine_expression