join_using = object() # tells mk_select() to join the column with USING

def mk_select(db, tables, fields=None, conds=None, limit=None, start=None,

    order_by=order_by_pkey, table_is_esc=False):

    '''

            right_col = table+'.'+esc_name_(right_col)

            sql_ = right_col+' '

            # Parse special values

            if left_col == None: left_col = (left_col,)

                # for None values, tuple is optional

            elif left_col == join_using: left_col = right_col

            # Create SQL

            if isinstance(left_col, tuple) and len(left_col) == 1:

                sql_ += ('= '+left_col+' OR ('+right_col+' IS NULL AND '

                    +left_col+' IS NULL)')

            return sql_

        if reduce(operator.and_, (v == join_using for v in joins.itervalues())):