# Django 2.1.8.dev20190330215650 documentation

# Source code for django.db.models.functions.comparison

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

    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_mysql(self, compiler, connection):
            # 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)

        def as_postgresql(self, compiler, connection):
            # 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')

    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):
            # 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':
                class ToNCLOB(Func):
                    function = 'TO_NCLOB'

                expressions = [
                    ToNCLOB(expression) for expression in self.get_source_expressions()
                ]
                clone = self.copy()
                clone.set_source_expressions(expressions)
                return super(Coalesce, clone).as_sql(compiler, connection)
            return self.as_sql(compiler, connection)

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

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