# Django 1.11.dev20170112000130 documentation

# Source code for django.db.models.expressions

    import copy
    import datetime

    from django.core.exceptions import EmptyResultSet, FieldError
    from django.db.backends import utils as backend_utils
    from django.db.models import fields
    from django.db.models.query_utils import Q
    from django.utils import six
    from django.utils.functional import cached_property

    class Combinable(object):
        """
        Provides the ability to combine one or two objects with
        some connector. For example F('foo') + F('bar').
        """

        # Arithmetic connectors
        ADD = '+'
        SUB = '-'
        MUL = '*'
        DIV = '/'
        POW = '^'
        # The following is a quoted % operator - it is quoted because it can be
        # used in strings that also have parameter substitution.
        MOD = '%%'

        # Bitwise operators - note that these are generated by .bitand()
        # and .bitor(), the '&' and '|' are reserved for boolean operator
        # usage.
        BITAND = '&'
        BITOR = '|'
        BITLEFTSHIFT = '<<'
        BITRIGHTSHIFT = '>>'

        def _combine(self, other, connector, reversed, node=None):
            if not hasattr(other, 'resolve_expression'):
                # everything must be resolvable to an expression
                if isinstance(other, datetime.timedelta):
                    other = DurationValue(other, output_field=fields.DurationField())
                else:
                    other = Value(other)

            if reversed:
                return CombinedExpression(other, connector, self)
            return CombinedExpression(self, connector, other)

        #############
        # OPERATORS #
        #############

        def __add__(self, other):
            return self._combine(other, self.ADD, False)

        def __sub__(self, other):
            return self._combine(other, self.SUB, False)

        def __mul__(self, other):
            return self._combine(other, self.MUL, False)

        def __truediv__(self, other):
            return self._combine(other, self.DIV, False)

        def __div__(self, other):  # Python 2 compatibility
            return type(self).__truediv__(self, other)

        def __mod__(self, other):
            return self._combine(other, self.MOD, False)

        def __pow__(self, other):
            return self._combine(other, self.POW, False)

        def __and__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

        def bitand(self, other):
            return self._combine(other, self.BITAND, False)

        def bitleftshift(self, other):
            return self._combine(other, self.BITLEFTSHIFT, False)

        def bitrightshift(self, other):
            return self._combine(other, self.BITRIGHTSHIFT, False)

        def __or__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

        def bitor(self, other):
            return self._combine(other, self.BITOR, False)

        def __radd__(self, other):
            return self._combine(other, self.ADD, True)

        def __rsub__(self, other):
            return self._combine(other, self.SUB, True)

        def __rmul__(self, other):
            return self._combine(other, self.MUL, True)

        def __rtruediv__(self, other):
            return self._combine(other, self.DIV, True)

        def __rdiv__(self, other):  # Python 2 compatibility
            return type(self).__rtruediv__(self, other)

        def __rmod__(self, other):
            return self._combine(other, self.MOD, True)

        def __rpow__(self, other):
            return self._combine(other, self.POW, True)

        def __rand__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

        def __ror__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

    class BaseExpression(object):
        """
        Base class for all query expressions.
        """

        # aggregate specific fields
        is_summary = False
        _output_field = None

        def __init__(self, output_field=None):
            if output_field is not None:
                self._output_field = output_field

        def get_db_converters(self, connection):
            return [self.convert_value] + self.output_field.get_db_converters(connection)

        def get_source_expressions(self):
            return []

        def set_source_expressions(self, exprs):
            assert len(exprs) == 0

        def _parse_expressions(self, *expressions):
            return [
                arg if hasattr(arg, 'resolve_expression') else (
                    F(arg) if isinstance(arg, six.string_types) else Value(arg)
                ) for arg in expressions
            ]

        def as_sql(self, compiler, connection):
            """
            Responsible for returning a (sql, [params]) tuple to be included
            in the current query.

            Different backends can provide their own implementation, by
            providing an `as_{vendor}` method and patching the Expression:

            ```
            def override_as_sql(self, compiler, connection):
                # custom logic
                return super(Expression, self).as_sql(compiler, connection)
            setattr(Expression, 'as_' + connection.vendor, override_as_sql)
            ```

            Arguments:
             * compiler: the query compiler responsible for generating the query.
               Must have a compile method, returning a (sql, [params]) tuple.
               Calling compiler(value) will return a quoted `value`.

             * connection: the database connection used for the current query.

            Returns: (sql, params)
              Where `sql` is a string containing ordered sql parameters to be
              replaced with the elements of the list `params`.
            """
            raise NotImplementedError("Subclasses must implement as_sql()")

        @cached_property
        def contains_aggregate(self):
            for expr in self.get_source_expressions():
                if expr and expr.contains_aggregate:
                    return True
            return False

        @cached_property
        def contains_column_references(self):
            for expr in self.get_source_expressions():
                if expr and expr.contains_column_references:
                    return True
            return False

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            """
            Provides the chance to do any preprocessing or validation before being
            added to the query.

            Arguments:
             * query: the backend query implementation
             * allow_joins: boolean allowing or denying use of joins
               in this query
             * reuse: a set of reusable joins for multijoins
             * summarize: a terminal aggregate clause
             * for_save: whether this expression about to be used in a save or update

            Returns: an Expression to be added to the query.
            """
            c = self.copy()
            c.is_summary = summarize
            c.set_source_expressions([
                expr.resolve_expression(query, allow_joins, reuse, summarize)
                for expr in c.get_source_expressions()
            ])
            return c

        def _prepare(self, field):
            """
            Hook used by Lookup.get_prep_lookup() to do custom preparation.
            """
            return self

        @property
        def field(self):
            return self.output_field

        @cached_property
        def output_field(self):
            """
            Returns the output type of this expressions.
            """
            if self._output_field_or_none is None:
                raise FieldError("Cannot resolve expression type, unknown output_field")
            return self._output_field_or_none

        @cached_property
        def _output_field_or_none(self):
            """
            Returns the output field of this expression, or None if no output type
            can be resolved. Note that the 'output_field' property will raise
            FieldError if no type can be resolved, but this attribute allows for
            None values.
            """
            if self._output_field is None:
                self._resolve_output_field()
            return self._output_field

        def _resolve_output_field(self):
            """
            Attempts to infer the output type of the expression. If the output
            fields of all source fields match then we can simply infer the same
            type here. This isn't always correct, but it makes sense most of the
            time.

            Consider the difference between `2 + 2` and `2 / 3`. Inferring
            the type here is a convenience for the common case. The user should
            supply their own output_field with more complex computations.

            If a source does not have an `_output_field` then we exclude it from
            this check. If all sources are `None`, then an error will be thrown
            higher up the stack in the `output_field` property.
            """
            if self._output_field is None:
                sources = self.get_source_fields()
                num_sources = len(sources)
                if num_sources == 0:
                    self._output_field = None
                else:
                    for source in sources:
                        if self._output_field is None:
                            self._output_field = source
                        if source is not None and not isinstance(self._output_field, source.__class__):
                            raise FieldError(
                                "Expression contains mixed types. You must set output_field")

        def convert_value(self, value, expression, connection, context):
            """
            Expressions provide their own converters because users have the option
            of manually specifying the output_field which may be a different type
            from the one the database returns.
            """
            field = self.output_field
            internal_type = field.get_internal_type()
            if value is None:
                return value
            elif internal_type == 'FloatField':
                return float(value)
            elif internal_type.endswith('IntegerField'):
                return int(value)
            elif internal_type == 'DecimalField':
                return backend_utils.typecast_decimal(value)
            return value

        def get_lookup(self, lookup):
            return self.output_field.get_lookup(lookup)

        def get_transform(self, name):
            return self.output_field.get_transform(name)

        def relabeled_clone(self, change_map):
            clone = self.copy()
            clone.set_source_expressions(
                [e.relabeled_clone(change_map) for e in self.get_source_expressions()])
            return clone

        def copy(self):
            c = copy.copy(self)
            c.copied = True
            return c

        def get_group_by_cols(self):
            if not self.contains_aggregate:
                return [self]
            cols = []
            for source in self.get_source_expressions():
                cols.extend(source.get_group_by_cols())
            return cols

        def get_source_fields(self):
            """
            Returns the underlying field types used by this
            aggregate.
            """
            return [e._output_field_or_none for e in self.get_source_expressions()]

        def asc(self, **kwargs):
            return OrderBy(self, **kwargs)

        def desc(self, **kwargs):
            return OrderBy(self, descending=True, **kwargs)

        def reverse_ordering(self):
            return self

        def flatten(self):
            """
            Recursively yield this expression and all subexpressions, in
            depth-first order.
            """
            yield self
            for expr in self.get_source_expressions():
                if expr:
                    for inner_expr in expr.flatten():
                        yield inner_expr

    class Expression(BaseExpression, Combinable):
        """
        An expression that can be combined with other expressions.
        """
        pass

    class CombinedExpression(Expression):

        def __init__(self, lhs, connector, rhs, output_field=None):
            super(CombinedExpression, self).__init__(output_field=output_field)
            self.connector = connector
            self.lhs = lhs
            self.rhs = rhs

        def __repr__(self):
            return "<{}: {}>".format(self.__class__.__name__, self)

        def __str__(self):
            return "{} {} {}".format(self.lhs, self.connector, self.rhs)

        def get_source_expressions(self):
            return [self.lhs, self.rhs]

        def set_source_expressions(self, exprs):
            self.lhs, self.rhs = exprs

        def as_sql(self, compiler, connection):
            try:
                lhs_output = self.lhs.output_field
            except FieldError:
                lhs_output = None
            try:
                rhs_output = self.rhs.output_field
            except FieldError:
                rhs_output = None
            if (not connection.features.has_native_duration_field and
                    ((lhs_output and lhs_output.get_internal_type() == 'DurationField') or
                     (rhs_output and rhs_output.get_internal_type() == 'DurationField'))):
                return DurationExpression(self.lhs, self.connector, self.rhs).as_sql(compiler, connection)
            if (lhs_output and rhs_output and self.connector == self.SUB and
                lhs_output.get_internal_type() in {'DateField', 'DateTimeField', 'TimeField'} and
                    lhs_output.get_internal_type() == lhs_output.get_internal_type()):
                return TemporalSubtraction(self.lhs, self.rhs).as_sql(compiler, connection)
            expressions = []
            expression_params = []
            sql, params = compiler.compile(self.lhs)
            expressions.append(sql)
            expression_params.extend(params)
            sql, params = compiler.compile(self.rhs)
            expressions.append(sql)
            expression_params.extend(params)
            # order of precedence
            expression_wrapper = '(%s)'
            sql = connection.ops.combine_expression(self.connector, expressions)
            return expression_wrapper % sql, expression_params

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            c.lhs = c.lhs.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            c.rhs = c.rhs.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

    class DurationExpression(CombinedExpression):
        def compile(self, side, compiler, connection):
            if not isinstance(side, DurationValue):
                try:
                    output = side.output_field
                except FieldError:
                    pass
                else:
                    if output.get_internal_type() == 'DurationField':
                        sql, params = compiler.compile(side)
                        return connection.ops.format_for_duration_arithmetic(sql), params
            return compiler.compile(side)

        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            expressions = []
            expression_params = []
            sql, params = self.compile(self.lhs, compiler, connection)
            expressions.append(sql)
            expression_params.extend(params)
            sql, params = self.compile(self.rhs, compiler, connection)
            expressions.append(sql)
            expression_params.extend(params)
            # order of precedence
            expression_wrapper = '(%s)'
            sql = connection.ops.combine_duration_expression(self.connector, expressions)
            return expression_wrapper % sql, expression_params

    class TemporalSubtraction(CombinedExpression):
        def __init__(self, lhs, rhs):
            super(TemporalSubtraction, self).__init__(lhs, self.SUB, rhs, output_field=fields.DurationField())

        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            lhs = compiler.compile(self.lhs, connection)
            rhs = compiler.compile(self.rhs, connection)
            return connection.ops.subtract_temporals(self.lhs.output_field.get_internal_type(), lhs, rhs)

    class F(Combinable):
        """
        An object capable of resolving references to existing query objects.
        """
        def __init__(self, name):
            """
            Arguments:
             * name: the name of the field this expression references
            """
            self.name = name

        def __repr__(self):
            return "{}({})".format(self.__class__.__name__, self.name)

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            return query.resolve_ref(self.name, allow_joins, reuse, summarize)

        def asc(self, **kwargs):
            return OrderBy(self, **kwargs)

        def desc(self, **kwargs):
            return OrderBy(self, descending=True, **kwargs)

    class Func(Expression):
        """
        An SQL function call.
        """
        function = None
        template = '%(function)s(%(expressions)s)'
        arg_joiner = ', '
        arity = None  # The number of arguments the function accepts.

        def __init__(self, *expressions, **extra):
            if self.arity is not None and len(expressions) != self.arity:
                raise TypeError(
                    "'%s' takes exactly %s %s (%s given)" % (
                        self.__class__.__name__,
                        self.arity,
                        "argument" if self.arity == 1 else "arguments",
                        len(expressions),
                    )
                )
            output_field = extra.pop('output_field', None)
            super(Func, self).__init__(output_field=output_field)
            self.source_expressions = self._parse_expressions(*expressions)
            self.extra = extra

        def __repr__(self):
            args = self.arg_joiner.join(str(arg) for arg in self.source_expressions)
            extra = ', '.join(str(key) + '=' + str(val) for key, val in self.extra.items())
            if extra:
                return "{}({}, {})".format(self.__class__.__name__, args, extra)
            return "{}({})".format(self.__class__.__name__, args)

        def get_source_expressions(self):
            return self.source_expressions

        def set_source_expressions(self, exprs):
            self.source_expressions = exprs

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            for pos, arg in enumerate(c.source_expressions):
                c.source_expressions[pos] = arg.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

        def as_sql(self, compiler, connection, function=None, template=None, arg_joiner=None, **extra_context):
            connection.ops.check_expression_support(self)
            sql_parts = []
            params = []
            for arg in self.source_expressions:
                arg_sql, arg_params = compiler.compile(arg)
                sql_parts.append(arg_sql)
                params.extend(arg_params)
            data = self.extra.copy()
            data.update(**extra_context)
            # Use the first supplied value in this order: the parameter to this
            # method, a value supplied in __init__()'s **extra (the value in
            # `data`), or the value defined on the class.
            if function is not None:
                data['function'] = function
            else:
                data.setdefault('function', self.function)
            template = template or data.get('template', self.template)
            arg_joiner = arg_joiner or data.get('arg_joiner', self.arg_joiner)
            data['expressions'] = data['field'] = arg_joiner.join(sql_parts)
            return template % data, params

        def as_sqlite(self, compiler, connection):
            sql, params = self.as_sql(compiler, connection)
            try:
                if self.output_field.get_internal_type() == 'DecimalField':
                    sql = 'CAST(%s AS NUMERIC)' % sql
            except FieldError:
                pass
            return sql, params

        def copy(self):
            copy = super(Func, self).copy()
            copy.source_expressions = self.source_expressions[:]
            copy.extra = self.extra.copy()
            return copy

    class Value(Expression):
        """
        Represents a wrapped value as a node within an expression
        """
        def __init__(self, value, output_field=None):
            """
            Arguments:
             * value: the value this expression represents. The value will be
               added into the sql parameter list and properly quoted.

             * output_field: an instance of the model field type that this
               expression will return, such as IntegerField() or CharField().
            """
            super(Value, self).__init__(output_field=output_field)
            self.value = value

        def __repr__(self):
            return "{}({})".format(self.__class__.__name__, self.value)

        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            val = self.value
            # check _output_field to avoid triggering an exception
            if self._output_field is not None:
                if self.for_save:
                    val = self.output_field.get_db_prep_save(val, connection=connection)
                else:
                    val = self.output_field.get_db_prep_value(val, connection=connection)
                if hasattr(self._output_field, 'get_placeholder'):
                    return self._output_field.get_placeholder(val, compiler, connection), [val]
            if val is None:
                # cx_Oracle does not always convert None to the appropriate
                # NULL type (like in case expressions using numbers), so we
                # use a literal SQL NULL
                return 'NULL', []
            return '%s', [val]

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = super(Value, self).resolve_expression(query, allow_joins, reuse, summarize, for_save)
            c.for_save = for_save
            return c

        def get_group_by_cols(self):
            return []

    class DurationValue(Value):
        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            if connection.features.has_native_duration_field:
                return super(DurationValue, self).as_sql(compiler, connection)
            return connection.ops.date_interval_sql(self.value)

    class RawSQL(Expression):
        def __init__(self, sql, params, output_field=None):
            if output_field is None:
                output_field = fields.Field()
            self.sql, self.params = sql, params
            super(RawSQL, self).__init__(output_field=output_field)

        def __repr__(self):
            return "{}({}, {})".format(self.__class__.__name__, self.sql, self.params)

        def as_sql(self, compiler, connection):
            return '(%s)' % self.sql, self.params

        def get_group_by_cols(self):
            return [self]

    class Star(Expression):
        def __repr__(self):
            return "'*'"

        def as_sql(self, compiler, connection):
            return '*', []

    class Random(Expression):
        def __init__(self):
            super(Random, self).__init__(output_field=fields.FloatField())

        def __repr__(self):
            return "Random()"

        def as_sql(self, compiler, connection):
            return connection.ops.random_function_sql(), []

    class Col(Expression):

        contains_column_references = True

        def __init__(self, alias, target, output_field=None):
            if output_field is None:
                output_field = target
            super(Col, self).__init__(output_field=output_field)
            self.alias, self.target = alias, target

        def __repr__(self):
            return "{}({}, {})".format(
                self.__class__.__name__, self.alias, self.target)

        def as_sql(self, compiler, connection):
            qn = compiler.quote_name_unless_alias
            return "%s.%s" % (qn(self.alias), qn(self.target.column)), []

        def relabeled_clone(self, relabels):
            return self.__class__(relabels.get(self.alias, self.alias), self.target, self.output_field)

        def get_group_by_cols(self):
            return [self]

        def get_db_converters(self, connection):
            if self.target == self.output_field:
                return self.output_field.get_db_converters(connection)
            return (self.output_field.get_db_converters(connection) +
                    self.target.get_db_converters(connection))

    class Ref(Expression):
        """
        Reference to column alias of the query. For example, Ref('sum_cost') in
        qs.annotate(sum_cost=Sum('cost')) query.
        """
        def __init__(self, refs, source):
            super(Ref, self).__init__()
            self.refs, self.source = refs, source

        def __repr__(self):
            return "{}({}, {})".format(self.__class__.__name__, self.refs, self.source)

        def get_source_expressions(self):
            return [self.source]

        def set_source_expressions(self, exprs):
            self.source, = exprs

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            # The sub-expression `source` has already been resolved, as this is
            # just a reference to the name of `source`.
            return self

        def relabeled_clone(self, relabels):
            return self

        def as_sql(self, compiler, connection):
            return "%s" % connection.ops.quote_name(self.refs), []

        def get_group_by_cols(self):
            return [self]

    class ExpressionWrapper(Expression):
        """
        An expression that can wrap another expression so that it can provide
        extra context to the inner expression, such as the output_field.
        """

        def __init__(self, expression, output_field):
            super(ExpressionWrapper, self).__init__(output_field=output_field)
            self.expression = expression

        def set_source_expressions(self, exprs):
            self.expression = exprs[0]

        def get_source_expressions(self):
            return [self.expression]

        def as_sql(self, compiler, connection):
            return self.expression.as_sql(compiler, connection)

        def __repr__(self):
            return "{}({})".format(self.__class__.__name__, self.expression)

    class When(Expression):
        template = 'WHEN %(condition)s THEN %(result)s'

        def __init__(self, condition=None, then=None, **lookups):
            if lookups and condition is None:
                condition, lookups = Q(**lookups), None
            if condition is None or not isinstance(condition, Q) or lookups:
                raise TypeError("__init__() takes either a Q object or lookups as keyword arguments")
            super(When, self).__init__(output_field=None)
            self.condition = condition
            self.result = self._parse_expressions(then)[0]

        def __str__(self):
            return "WHEN %r THEN %r" % (self.condition, self.result)

        def __repr__(self):
            return "<%s: %s>" % (self.__class__.__name__, self)

        def get_source_expressions(self):
            return [self.condition, self.result]

        def set_source_expressions(self, exprs):
            self.condition, self.result = exprs

        def get_source_fields(self):
            # We're only interested in the fields of the result expressions.
            return [self.result._output_field_or_none]

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            if hasattr(c.condition, 'resolve_expression'):
                c.condition = c.condition.resolve_expression(query, allow_joins, reuse, summarize, False)
            c.result = c.result.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

        def as_sql(self, compiler, connection, template=None, **extra_context):
            connection.ops.check_expression_support(self)
            template_params = extra_context
            sql_params = []
            condition_sql, condition_params = compiler.compile(self.condition)
            template_params['condition'] = condition_sql
            sql_params.extend(condition_params)
            result_sql, result_params = compiler.compile(self.result)
            template_params['result'] = result_sql
            sql_params.extend(result_params)
            template = template or self.template
            return template % template_params, sql_params

        def get_group_by_cols(self):
            # This is not a complete expression and cannot be used in GROUP BY.
            cols = []
            for source in self.get_source_expressions():
                cols.extend(source.get_group_by_cols())
            return cols

    class Case(Expression):
        """
        An SQL searched CASE expression:

            CASE
                WHEN n > 0
                    THEN 'positive'
                WHEN n < 0
                    THEN 'negative'
                ELSE 'zero'
            END
        """
        template = 'CASE %(cases)s ELSE %(default)s END'
        case_joiner = ' '

        def __init__(self, *cases, **extra):
            if not all(isinstance(case, When) for case in cases):
                raise TypeError("Positional arguments must all be When objects.")
            default = extra.pop('default', None)
            output_field = extra.pop('output_field', None)
            super(Case, self).__init__(output_field)
            self.cases = list(cases)
            self.default = self._parse_expressions(default)[0]
            self.extra = extra

        def __str__(self):
            return "CASE %s, ELSE %r" % (', '.join(str(c) for c in self.cases), self.default)

        def __repr__(self):
            return "<%s: %s>" % (self.__class__.__name__, self)

        def get_source_expressions(self):
            return self.cases + [self.default]

        def set_source_expressions(self, exprs):
            self.cases = exprs[:-1]
            self.default = exprs[-1]

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            for pos, case in enumerate(c.cases):
                c.cases[pos] = case.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            c.default = c.default.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

        def copy(self):
            c = super(Case, self).copy()
            c.cases = c.cases[:]
            return c

        def as_sql(self, compiler, connection, template=None, case_joiner=None, **extra_context):
            connection.ops.check_expression_support(self)
            if not self.cases:
                return compiler.compile(self.default)
            template_params = self.extra.copy()
            template_params.update(extra_context)
            case_parts = []
            sql_params = []
            for case in self.cases:
                try:
                    case_sql, case_params = compiler.compile(case)
                except EmptyResultSet:
                    continue
                case_parts.append(case_sql)
                sql_params.extend(case_params)
            default_sql, default_params = compiler.compile(self.default)
            if not case_parts:
                return default_sql, default_params
            case_joiner = case_joiner or self.case_joiner
            template_params['cases'] = case_joiner.join(case_parts)
            template_params['default'] = default_sql
            sql_params.extend(default_params)
            template = template or template_params.get('template', self.template)
            sql = template % template_params
            if self._output_field_or_none is not None:
                sql = connection.ops.unification_cast_sql(self.output_field) % sql
            return sql, sql_params

    class OrderBy(BaseExpression):
        template = '%(expression)s %(ordering)s'

        def __init__(self, expression, descending=False, nulls_first=False, nulls_last=False):
            if nulls_first and nulls_last:
                raise ValueError('nulls_first and nulls_last are mutually exclusive')
            self.nulls_first = nulls_first
            self.nulls_last = nulls_last
            self.descending = descending
            if not hasattr(expression, 'resolve_expression'):
                raise ValueError('expression must be an expression type')
            self.expression = expression

        def __repr__(self):
            return "{}({}, descending={})".format(
                self.__class__.__name__, self.expression, self.descending)

        def set_source_expressions(self, exprs):
            self.expression = exprs[0]

        def get_source_expressions(self):
            return [self.expression]

        def as_sql(self, compiler, connection, template=None, **extra_context):
            if not template:
                if self.nulls_last:
                    template = '%s NULLS LAST' % self.template
                elif self.nulls_first:
                    template = '%s NULLS FIRST' % self.template
            connection.ops.check_expression_support(self)
            expression_sql, params = compiler.compile(self.expression)
            placeholders = {
                'expression': expression_sql,
                'ordering': 'DESC' if self.descending else 'ASC',
            }
            placeholders.update(extra_context)
            template = template or self.template
            return (template % placeholders).rstrip(), params

        def as_sqlite(self, compiler, connection):
            template = None
            if self.nulls_last:
                template = '%(expression)s IS NULL, %(expression)s %(ordering)s'
            elif self.nulls_first:
                template = '%(expression)s IS NOT NULL, %(expression)s %(ordering)s'
            return self.as_sql(compiler, connection, template=template)

        def as_mysql(self, compiler, connection):
            template = None
            if self.nulls_last:
                template = 'IF(ISNULL(%(expression)s),1,0), %(expression)s %(ordering)s '
            elif self.nulls_first:
                template = 'IF(ISNULL(%(expression)s),0,1), %(expression)s %(ordering)s '
            return self.as_sql(compiler, connection, template=template)

        def get_group_by_cols(self):
            cols = []
            for source in self.get_source_expressions():
                cols.extend(source.get_group_by_cols())
            return cols

        def reverse_ordering(self):
            self.descending = not self.descending
            return self

        def asc(self):
            self.descending = False

        def desc(self):
            self.descending = True