MySQL 8.0

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create table loans  (book_id int, student_id int, date_taken date, number_od_days int);
insert into loans values (1, 201, '2018/10/02', 5);
insert into loans values (2, 202, '2018/10/10', 10);
insert into loans values (9, 203, '2018/10/25', 1);
insert into loans values (3, 204, '2018/10/30', 20);
insert into loans values (7, 205, '2018/12/02', 11);
select book_id
       , student_id
       , date_taken
       , number_od_days
       , (date_taken + interval number_od_days day) as return_date
from loans;


book_id
student_id
date_taken
number_od_days
return_date


1
201
2018-10-02
5
2018-10-07

2
202
2018-10-10
10
2018-10-20

9
203
2018-10-25
1
2018-10-26

3
204
2018-10-30
20
2018-11-19

7
205
2018-12-02
11
2018-12-13

book_id	student_id	date_taken	number_od_days	return_date
1	201	2018-10-02	5	2018-10-07
2	202	2018-10-10	10	2018-10-20
9	203	2018-10-25	1	2018-10-26
3	204	2018-10-30	20	2018-11-19
7	205	2018-12-02	11	2018-12-13