SQL Server 2017

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create table dbo.grades(student_name nvarchar(20) not NULL, grades int);
insert into dbo.grades
VALUES
(   'Ann',      NULL),
(   'Bob',     NULL),
(   'Bob',     10),
(   'Jane',      25),
(   'Nelly',     30);
5 rows affected

SELECT * 
FROM dbo.grades 
ORDER BY CASE WHEN grades IS NULL 
THEN 2 ELSE 1 END ASC, grades ASC;


student_name
grades


Bob
10

Jane
25

Nelly
30

Ann
null

Bob
null


SELECT * 
FROM dbo.grades 
ORDER BY -grades DESC;


student_name
grades


Bob
10

Jane
25

Nelly
30

Ann
null

Bob
null