Generating a date in a table SQL - mysql

Question

I have a database in SQL which contains information about students and books they are borrowing.
I have a table of loans which includes the book_id, the student_id, the day the book was taken and the number of days the book can be kept.
I want to auto-generate the return_date by adding the days at the date in which the book was taken. How can I do this using MySQL Workbench?

Answer 1

You can use MySQL datetime function date_add() to compute the target return_date. Assuming that the date when the book was loaned is called loan_date and that the duration is stored in loan_duration, expressed in days:
select
book_id,
student_id,
loan_date,
date_add(loan_date, interval loan_duration day) return_date
from loan_table
In an update statement:
update loan_table
set return_date = date_add(loan_date, interval loan_duration day)
where return_date is null -- only compute the return_dates that were not yet computed
Edit
If you want that value to be auto-generated, an option is to use a generated stored (or virtual) column (available since MySQL 5.7). This works by defining a computation rule, that MySQL will automatically apply when a record is inserted or updated.
Demo on DB Fiddle:
-- set up
create table loan_table(
id int primary key auto_increment,
book_id int,
student_id int,
loan_date date,
loan_duration int,
-- here is the generated column
return_date date
as (date_add(loan_date, interval loan_duration day)) stored
);
-- perform an insert
insert into loan_table(book_id, student_id, loan_date, loan_duration)
values(1, 1, '2019-11-02', 3)
-- check the computed value
select * from loan_table;
id | book_id | student_id | loan_date | loan_duration | return_date
-: | ------: | ---------: | :--------- | ------------: | :----------
1 | 1 | 1 | 2019-11-02 | 3 | 2019-11-05

Answer 2

You can use a combination of interval and + :
select book_id
, student_id
, date_taken
, number_od_days
, (date_taken + interval number_od_days day) as return_date
from loans;
Here is the example in the DEMO
If you want this column to be auto incremented I suggest you create a trigger in your table:
CREATE TRIGGER rentcheck
BEFORE INSERT ON loans
FOR EACH ROW
begin
if new.number_od_days < 0 or new.number_od_days is null then
signal sqlstate '45000';
else
set new.return_date = (new.date_taken + interval new.number_od_days day);
end if;
end;
With it you can control if the column number_of_days will be negative or not entered because that would result with non logical data. Here is the demo for the trigger: DEMO

Answer 3

you can use variables in mysql:
set #days = 10;
set #mydate := DATE_ADD("2019-11-01", INTERVAL #days DAY);
select #mydate;
and then use your insert statement:
Insert into Loan(... ,[return_date])
values (...., #mydate)
Hope it helps