def to_str(self, db): return self.str_

class Literal(Code):

    def __init__(self, value): self.value = value

def is_null(value): return isinstance(value, Literal) and value.value == None

class Table(Code):

    def __init__(self, name, schema=None):

        '''

    if table == None or isinstance(table, Code): return table

    else: return Table(table)

class Col(Code):

    def __init__(self, name, table=None):

        '''

    def to_Col(self): return Col(self.name)

class ValueCond:

    def __init__(self, value):

        return CompareCond(value)

    else: return value

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

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