Postgres 13

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CREATE TABLE pupil
    (id integer primary key,
     marks integer,
     maxmarks integer);
     
insert into pupil (id, marks, maxmarks) values (1,11,100),(2,21,100),(3, 60,100);
3 rows affected

SELECT
    CASE 
        WHEN marks <= 30 THEN '0-30'
        ELSE '31-100'
    END,
    COUNT(*)
FROM 
    pupil
GROUP BY (marks <= 30)


case
count


31-100
1

0-30
2


SELECT
    '0-30',
    COUNT(*)
FROM 
    pupil
WHERE marks <= 30

UNION

SELECT
    '31-100',
    COUNT(*)
FROM 
    pupil
WHERE marks > 30


?column?
count


0-30
2

31-100
1