MySQL 8.0

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CREATE  TABLE reasons
(
Reasons varchar(8) ,
Counts_students VARCHAR(50) ,
total_count int
);


INSERT INTO reasons
VALUES ( 'A',2,null),
( 'B',3,null),
('C',8,null),
('D',1,null),
('E',2,null),
('F',5,null),
(NULL,NULL,45)
SELECT reasons,Counts_students FROM reasons WHERE reasons IS NOT NULL
UNION SELECT 'Rest', (SELECT MAX(total_count) - SUM(Counts_students) FROM reasons)


reasons
Counts_students


A
2

B
3

C
8

D
1

E
2

F
5

Rest
24