Sysdate+days as default value in table column - Oracle - oracle

Question

I'm working on my table which is supposed to store data about rented cars.
And there are 3 important columns:
RENT_DATE DATE DEFAULT TO_DATE (SYSDATE, 'DD-MM-YYYY'),
DAYS NUMBER NOT NULL,
RETURN_DATE DATE DEFAULT TO_DATE(SYSDATE+DAYS, 'DD-MM-YYYY')
My problem is that RETURN_DATE column is giving me error:
00984. 00000 - "column not allowed here"
What i want is that RENT_DATE set automatically date when record is added.
DAYS column is to store for how much days someone is renting car.
And the last column should store date of when car should be returned.
Thank you for any type of help.

Answer 1

This doesn't make sense:
DEFAULT TO_DATE (SYSDATE, 'DD-MM-YYYY')
SYSDATE is already a date. TO_DATE requires a char, so this takes a date, Oracle implicitly turns the date into a char, and then TO_DATE converts it back to a date. This is risky/unreliable because it uses a hardcoded date format to operate on a date that has been implicitly turned to a string using the system default format, which might one day not be DD-MM-YYYY (you're building a latent bug into your software)
If you want a date without a time on it use TRUNC(SYSDATE)
The other problem doesn't make sense either: you're storing a number of days rented for and also the return date, when one is a function of the other. Storing redundant data becomes a headache because you have to keep them in sync. My person class stores my birthdate, and I calculate how old I am. I don't store my age too and then update my table every day/year etc
Work out which will be more beneficial to you to store, and store it, then calculate the other whenever you want it. Personally I would store the return date as it's absolute, rather than open to interpretation of "is that working days, calendar days? what about public holidays? if the start date is jan 1 and the rental is for 10 days, is the car brought back on the 10th or the 11th?"
If you're desperate to have both columns in your DB consider using a view to calculate it or a function based column (again, to calculate one from the other) so they stay in sync
All in, you could look at this:
create table X(
RENT_DATE DATE DEFAULT TRUNC(SYSDATE) NOT NULL,
RETURN_DATE DATE NOT NULL,
DAYS AS (TRUNC(RETURN_DATE - RENT_DATE) + 1)
)
I put the days as +1 because to me, a car taken on the 1st and returned on the second is 2 days, but you might want to get more accurate - if it's taken on the first and returned before 10am on the second then it's one day otherwise it's 2 etc...

Answer 2

Use a virtual column:
CREATE TABLE table_name (
RENT_DATE DATE
DEFAULT TRUNC( SYSDATE )
CONSTRAINT table_name__rent_date__nn NOT NULL
CONSTRAINT table_name__rent_date_chk CHECK ( rent_date = TRUNC( rent_date ) ),
DAYS NUMBER
DEFAULT 7
CONSTRAINT table_name__days__nn NOT NULL,
RETURN_DATE DATE
GENERATED ALWAYS AS ( RENT_DATE + DAYS ) VIRTUAL
);
Then you can insert values:
INSERT INTO table_name ( rent_date, days ) VALUES ( DEFAULT, DEFAULT );
INSERT INTO table_name ( rent_date, days ) VALUES ( DATE '2020-01-01', 1 );
And:
SELECT * FROM table_name;
Outputs:
RENT_DATE | DAYS | RETURN_DATE
:------------------ | ---: | :------------------
2020-09-12T00:00:00 | 7 | 2020-09-19T00:00:00
2020-01-01T00:00:00 | 1 | 2020-01-02T00:00:00
db<>fiddle here