Creating a column with a specified date/time format on Oracle + - oracle

Question

I am working on a set of tables for an assignment related to airports,
CREATE TABLE FLIGHT
(FlightCode VARCHAR2(6),
AirplaneSerialNum VARCHAR2(6) NOT NULL,
RemCapacity NUMBER(3),
FlightDate DATE NOT NULL,
RouteNum NUMBER(5) NOT NULL,
FlightDeparture DATE,
FlightArrival DATE,
CONSTRAINT FlightPK PRIMARY KEY (FlightCode),
CONSTRAINT RouteFK FOREIGN KEY (RouteNum) REFERENCES ROUTE (RouteNum));
Where FlightDeparture and FlightArrival are ideally referenced in the 24H format or 12H format (HHMM), how should I go about writing it, and how can I make a column in a separate table referencing the difference (to 2dp.) in number of hours between FlightArrival and FlightDeparture?
e.g. in another table that references FlightCode in FLIGHT as a FK
HoursInFlight NUMBER(10,2)
where HoursInFlight = FlightArrival - FlightDeparture

Answer 1

DATE datatype looks OK; it stores values in Oracle's internal format, so it is up to you how will you display them to users. For example:
Inserting:
SQL> create table flight (flight_date date);
Table created.
SQL> insert into flight (flight_date) values (sysdate);
1 row created.
SQL> insert into flight (flight_date) values (to_date('18.07.2021 16:58', 'dd.mm.yyyy hh24:Mi'));
1 row created.
Selecting (various options):
SQL> select * from flight;
FLIGHT_D
--------
18.07.21
18.07.21
SQL> select to_char(flight_date, 'dd-mon-yy hh:mi am', 'nls_date_language = english')) val from flight;
select to_char(flight_date, 'dd-mon-yy hh:mi am', 'nls_date_language = english')) val from flight
*
ERROR at line 1:
ORA-00923: FROM keyword not found where expected
SQL> select to_char(flight_date, 'dd-mon-yy hh:mi am', 'nls_date_language = english') val from flight;
VAL
---------------------------
18-jul-21 04:58 PM
18-jul-21 04:58 PM
SQL> select to_char(flight_date, 'dd/mm/yyyy hh24:mi:ss') val from flight;
VAL
-------------------
18/07/2021 16:58:36
18/07/2021 16:58:00
SQL> alter session set nls_Date_Format = 'yyyy-mm-dd hh:mi am';
Session altered.
SQL> select * from flight;
FLIGHT_DATE
-------------------
2021-07-18 04:58 PM
2021-07-18 04:58 PM
SQL>
The difference of two DATE values is number of days between them, so - if you want to "convert" them to hours, multiply it by 24 (as there are 24 hours in a day):
SQL> select to_date('18.07.2021 17:02', 'dd.mm.yyyy hh24:mi') - to_date('18.07.2021 10:44', 'dd.mm.yyyy hh24:mi') diff
2 from dual;
DIFF
----------
,2625
SQL> select 0.2625 * 24 hours from dual;
HOURS
----------
6,3
SQL>