# Django 2.2.9.dev20191202104323 文档

# django.db.models.functions.comparison 源代码

    """Database functions that do comparisons or type conversions."""
    from django.db.models.expressions import Func, Value

    class Cast(Func):
        """Coerce an expression to a new field type."""
        function = 'CAST'
        template = '%(function)s(%(expressions)s AS %(db_type)s)'

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

        def as_sql(self, compiler, connection, **extra_context):
            extra_context['db_type'] = self.output_field.cast_db_type(connection)
            return super().as_sql(compiler, connection, **extra_context)

        def as_sqlite(self, compiler, connection, **extra_context):
            db_type = self.output_field.db_type(connection)
            if db_type in {'datetime', 'time'}:
                # Use strftime as datetime/time don't keep fractional seconds.
                template = 'strftime(%%s, %(expressions)s)'
                sql, params = super().as_sql(compiler, connection, template=template, **extra_context)
                format_string = '%H:%M:%f' if db_type == 'time' else '%Y-%m-%d %H:%M:%f'
                params.insert(0, format_string)
                return sql, params
            elif db_type == 'date':
                template = 'date(%(expressions)s)'
                return super().as_sql(compiler, connection, template=template, **extra_context)
            return self.as_sql(compiler, connection, **extra_context)

        def as_mysql(self, compiler, connection, **extra_context):
            # MySQL doesn't support explicit cast to float.
            template = '(%(expressions)s + 0.0)' if self.output_field.get_internal_type() == 'FloatField' else None
            return self.as_sql(compiler, connection, template=template, **extra_context)

        def as_postgresql(self, compiler, connection, **extra_context):
            # CAST would be valid too, but the :: shortcut syntax is more readable.
            # 'expressions' is wrapped in parentheses in case it's a complex
            # expression.
            return self.as_sql(compiler, connection, template='(%(expressions)s)::%(db_type)s', **extra_context)

    class Coalesce(Func):
        """Return, from left to right, the first non-null expression."""
        function = 'COALESCE'

        def __init__(self, *expressions, **extra):
            if len(expressions) < 2:
                raise ValueError('Coalesce must take at least two expressions')
            super().__init__(*expressions, **extra)

        def as_oracle(self, compiler, connection, **extra_context):
            # Oracle prohibits mixing TextField (NCLOB) and CharField (NVARCHAR2),
            # so convert all fields to NCLOB when that type is expected.
            if self.output_field.get_internal_type() == 'TextField':
                clone = self.copy()
                clone.set_source_expressions([
                    Func(expression, function='TO_NCLOB') for expression in self.get_source_expressions()
                ])
                return super(Coalesce, clone).as_sql(compiler, connection, **extra_context)
            return self.as_sql(compiler, connection, **extra_context)

    class Greatest(Func):
        """
        Return the maximum expression.

        If any expression is null the return value is database-specific:
        On PostgreSQL, the maximum not-null expression is returned.
        On MySQL, Oracle, and SQLite, if any expression is null, null is returned.
        """
        function = 'GREATEST'

        def __init__(self, *expressions, **extra):
            if len(expressions) < 2:
                raise ValueError('Greatest must take at least two expressions')
            super().__init__(*expressions, **extra)

        def as_sqlite(self, compiler, connection, **extra_context):
            """Use the MAX function on SQLite."""
            return super().as_sqlite(compiler, connection, function='MAX', **extra_context)

    class Least(Func):
        """
        Return the minimum expression.

        If any expression is null the return value is database-specific:
        On PostgreSQL, return the minimum not-null expression.
        On MySQL, Oracle, and SQLite, if any expression is null, return null.
        """
        function = 'LEAST'

        def __init__(self, *expressions, **extra):
            if len(expressions) < 2:
                raise ValueError('Least must take at least two expressions')
            super().__init__(*expressions, **extra)

        def as_sqlite(self, compiler, connection, **extra_context):
            """Use the MIN function on SQLite."""
            return super().as_sqlite(compiler, connection, function='MIN', **extra_context)

    class NullIf(Func):
        function = 'NULLIF'
        arity = 2

        def as_oracle(self, compiler, connection, **extra_context):
            expression1 = self.get_source_expressions()[0]
            if isinstance(expression1, Value) and expression1.value is None:
                raise ValueError('Oracle does not allow Value(None) for expression1.')
            return super().as_sql(compiler, connection, **extra_context)