Compile the "from" clause for an update with a join.

        :param query: A QueryBuilder instance
        :type query: QueryBuilder

        :return: The compiled sql
        :rtype: str

def _compile_update_from(self, query):
        """
        Compile the "from" clause for an update with a join.

        :param query: A QueryBuilder instance
        :type query: QueryBuilder

        :return: The compiled sql
        :rtype: str
        """
        if not query.joins:
            return ""

        froms = []

        for join in query.joins:
            froms.append(self.wrap_table(join.table))

        if len(froms):
            return " FROM %s" % ", ".join(froms)

        return ""