##### Joins

filter_out = object() # tells Join to filter out rows that match the join

    def __init__(self, table, mapping, type_=None):

        '''

        @param mapping dict(right_table_col=left_table_col, ...)

        @param type_ None (for plain join)|str (e.g. 'LEFT')|filter_out

            * filter_out: equivalent to 'LEFT' with the query filtered by

              `table_pkey IS NULL` (indicating no match)

            # Note that right_table_col is on the left in the comparison

            # Parse special values

            cond = as_ValueCond(left_table_col, left_table)

            return cond.to_str(db, as_Col(right_table_col, self.table))

        type_ = self.type_

        joins = self.mapping

        if type_ is not filter_out and reduce(operator.and_,

            # all cols w/ USING, so can use simpler USING syntax

            cols = (as_Col(v).to_str(db) for v in joins.iterkeys())

            join_cond = 'USING ('+(', '.join(cols))+')'