如果要查找所有带有null in标记的行，请使用：

SELECT DISTINCT Student,Class
FROM report
WHERE Marks IS NULL;

..distinct运算符从结果中删除重复项
另一种变体是：

SELECT Student,Class
FROM report
GROUP BY Student,Class
HAVING COUNT(*)=COUNT(*)FILTER(WHERE Marks IS NULL)

我为你的问题举了个例子。

CREATE TABLE reportt (
  Class  int,
  Marks  int,
  Student  VARCHAR(100),
  Observation_time VARCHAR(100),
);

INSERT INTO reportt
  (Student, Class, Marks,Observation_time)
VALUES
  ('A',1,11,'21/7/2020'),
  ('A',2,13,'18/7/2020'),
  ('B',1,19,'17/7/2020'),
  ('A',1,17,'15/7/2020'),
  ('B',1,15,'21/7/2020'),
  ('C',1,null,'10/7/2015'),
  ('C',1,null,'11/7/2015'),
  ('C',2,8,'10/7/2015'),
  ('C',2,0,'11/7/2015'),
  ('D',1,null,'10/7/2015'),
  ('D',1,null,'11/7/2015'),
  ('D',2,null,'10/7/2015'),
  ('D',2,null,'11/7/2015')
  ;

with CTE_select as (
    select ISNULL(Marks,0) Marks, Student,Class 
    from  reportt 
  )
  select Student,Class,SUM(Marks) from CTE_select
  where marks >= 0
  group by Class,Student
  having   SUM(Marks)= 0;

结果=

Student Class
C        1  
D        1  
D        2

查看关于group by的文档，它非常强大，但也可能相当棘手。前面的答案（distinct）实际上也是一种groupby。我认为这应该得到你想要的结果，但请阅读文件，以了解正在发生的事情。

Select MIN(Student), MIN(Class)
  from report
where Marks = 0
  group by Student, Class