# SQL code generation

    def to_str(self, db): raise NotImplemented()

    def __str__(self): return str(self.__dict__)

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

    def to_str(self, db): return db.esc_value(self.value)

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

        '''

        @param schema str|None (for no schema)

        '''

        self.name = name

        self.schema = schema

    def to_str(self, db): return sql.qual_name(db, self.schema, self.name)

        schema, table = table

        return Table(table, schema)

    else: return Table(table)

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

        '''

        @param table Table|None (for no table)

        '''

        self.name = name

        self.table = table

    def to_str(self, db):

        str_ = ''

        if self.table != None: str_ += self.table.to_str(db)+'.'

        str_ += sql.esc_name(db, self.name)

        return str_

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

    else: return Col(col, table)

    def __init__(self, name, code):

        if not isinstance(code, Code): code = Literal(code)

        self.name = name

        self.code = code

    def to_str(self, db):

        return self.code.to_str(db)+' AS '+sql.esc_name(db, self.name)

        self.value = value

        raise NotImplemented()

    def __str__(self): return str(self.__dict__)

class CompareCond(ValueCond):

    def __init__(self, value, operator='='):

        @param operator By default, compares NULL values literally. Use '~=' or

            '~!=' to pass NULLs through.

        '''

        self.operator = operator

        if not isinstance(left_value, Code): left_value = Col(left_value)

        left = left_value.to_str(db)

        right = right_value.to_str(db)

        # Parse operator

        operator = strings.remove_prefix('~', operator, passthru_null_ref)

        neg_ref = [False]

        operator = strings.remove_prefix('!', operator, neg_ref)

        equals = operator.endswith('=')

        if equals and is_null(self.value): operator = 'IS'

        # Create str

        str_ = left+' '+operator+' '+right

        if equals and not passthru_null_ref[0] and isinstance(right_value, Col):

            str_ += ' OR ('+left+' IS NULL AND '+right+' IS NULL)'

        if neg_ref[0]: str_ = 'NOT ('+str_+')'

        return str_

assume_literal = object()

def as_ValueCond(value, default_table=assume_literal):

    if not isinstance(value, ValueCond):

        if default_table is not assume_literal:

            value = as_Col(value, default_table)

        return CompareCond(value)

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

class Join(Code):

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

        '''

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

            * if left_table_col is join_using: left_table_col = right_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)

        '''

        if util.is_str(table): table = Table(table)

        assert type_ == None or util.is_str(type_) or type_ is filter_out

        self.table = table

        self.mapping = mapping

        self.type_ = type_

    def to_str(self, db, left_table):

        def join(entry):

            '''Parses non-USING joins'''

            right_table_col, left_table_col = entry

            # Parse special values

            if left_table_col is join_using: left_table_col = right_table_col

            cond = as_ValueCond(right_table_col, self.table)

            return cond.to_str(db, as_Col(left_table_col, left_table))

        type_ = self.type_

            (v is join_using for v in joins.itervalues())):

            join_cond = 'USING ('+(', '.join(joins.iterkeys()))+')'

        else: join_cond = 'ON '+(' AND '.join(map(join, joins.iteritems())))

        # Create join

        if type_ is filter_out: type_ = 'LEFT'

        if type_ != None: str_ += type_+' '

def unescape_table(table):

    '''Currently only works with PostgreSQL.'''

    if table == None: return table

    assert table.count('.') <= 1

    parts = tuple((v.replace('"', '') for v in table.split('"."', 2)))

    if len(parts) == 1: parts, = parts

    return parts

    '''Converts old-style (tuple-based) tables to sql_gen-compatible values.

    @param table_is_esc If False, assumes any table name is not escaped or that

        re-escaping it will produce the same value.

    '''

    if util.is_str(table) and table_is_esc: table = unescape_table(table)

    return as_Table(table)

        re-escaping it will produce the same value.

    '''

    if isinstance(col, Col): return col # already in sql_gen form

    table = default_table

    if isinstance(col, tuple): table, col = col

    assume_col=False):

    @param table_is_esc If False, assumes any table name is not escaped or that

        re-escaping it will produce the same value.

    '''

    if isinstance(value, Code): return value # already in sql_gen form

    if is_tuple and len(value) == 1: return Literal(value[0])

    if is_tuple or (assume_col and util.is_str(value)):

        return col2sql_gen(value, default_table, table_is_esc)

    else: return Literal(value)