distinct_on_all = object() # tells mk_select() to SELECT DISTINCT ON all columns

def mk_select(db, tables, fields=None, conds=None, distinct_on=[], limit=None,

    '''

    @param tables The single table to select from, or a list of tables to join

        together, with tables after the first being sql_gen.Join objects

        * compare_right_side: ValueCond|literal value

    @param distinct_on The columns to SELECT DISTINCT ON, or distinct_on_all to

        use all columns

    @return tuple(query, params)

    '''

    def esc_name_(name): return esc_name(db, name)

    if conds == None: conds = {}

    assert limit == None or type(limit) == int

    assert start == None or type(start) == int

    if order_by is order_by_pkey: order_by = pkey(db, table0, recover=True)

    params = []