Question

I have a start_session and end_session.

I want to make sure with a constraint that the end_session is 60 minutes after the start_session.

Is this possible, something like the following

ALTER TABLE BOOKING_SESSION ADD CONSTRAINT SESSION_LENGHT CHECK (END_SESSION > START_SESSION + 60 MINUTES );

I got the following trigger to check that the start session is not bigger than the end session (thanks to Justin Cave)

CREATE OR REPLACE TRIGGER trg_check_dates
  BEFORE INSERT OR UPDATE ON BOOKING_SESSION
  FOR EACH ROW
BEGIN
  IF( :NEW.END_SESSION < :NEW.START_SESSION )
  THEN
    RAISE_APPLICATION_ERROR( -20001, 
          'Invalid: END_SESSION must be greater than START_SESSION = ' || 
          to_char( :NEW.START_SESSION, 'YYYY-MM-DD HH24:MI:SS' ));
  END IF;
 END;
 /

However i need the one that makes sure that the end session is one hour after the start_session

Answer 1

Here is how the Check constraint can be used to impose that kind of restriction:

SQL> create table Tb_test(
  2    id number not null primary key,
  3    start_session timestamp,
  4    end_session   timestamp
  5  )
  6  ;

Table created

SQL> 
SQL> alter table tb_test
  2    add constraint CHK_SES_TIME
  3  check ((end_session is null) or ((end_session - start_session) >= interval '1' hour))
  4  ;

Table altered

-- data that is violate our constraint
SQL> insert into tb_test(id, start_session, end_session)
  2    values(1, systimestamp, systimestamp)
  3  ;

insert into tb_test(id, start_session, end_session)
  values(1, systimestamp, systimestamp)

ORA-02290: check constraint (HR.CHK_SES_TIME) violated

-- valid data 
SQL> insert into tb_test(id, start_session, end_session)
  2    values(1, systimestamp, systimestamp + interval '1' hour)
  3  ;

1 row inserted 

-- only start session added, end_session is null.
SQL> insert into tb_test(id, start_session)
  2    values(2, systimestamp)
  3  ;

1 row inserted

SQL> commit;

Commit complete

-- updating end_session with data that violate check constraint
SQL> update tb_test
  2    set end_session = systimestamp;

update tb_test
  set end_session = systimestamp

ORA-02290: check constraint (HR.CHK_SES_TIME) violated

-- updating end_session with valid data


SQL> update tb_test
  2    set end_session = systimestamp + interval '1' hour;

 1 row updated

In the example above start_session and end_session columns are of timestamp datatype. If you have them declared as Date datatype then formula for the check constraint might be:

(end_session - srtart_session) >= 1/24