Question

I am trying to create a trigger to calculate a derived attribute on each insert command. However I am getting compilation errors, I dont know where is the problem.

CREATE OR REPLACE TRIGGER NewTrigger 
BEFORE INSERT 
ON Dates FOR EACH ROW 
 BEGIN 
   SET :NEW.difference := :NEW.date1 - :NEW.date2; 
 END;

Show errors shows me this information:

LINE/COL ERROR
-------- -----------------------------------------------------------------
1/7  PL/SQL: SQL Statement ignored
1/11     PL/SQL: ORA-00922: missing or invalid option

Answer 1

It’s not the trigger, it’s the data type. If you substract a date from another date, the result is an interval, not another date:

CREATE TABLE dates (date1 DATE, date2 DATE, datediff DATE, numdiff NUMBER);
INSERT INTO dates (date1, date2) VALUES (sysdate, sysdate-1);

UPDATE dates SET numdiff = date1 - date2;
1 rows updated

UPDATE dates SET datediff = date1 - date2;
SQL Error: ORA-00932: inconsistent datatypes: expected DATE got DATE JULIAN

So, if the trigger stores the interval in a number, it compiles:

CREATE OR REPLACE TRIGGER newtriggernum
  BEFORE INSERT ON dates FOR EACH ROW
BEGIN
  :new.numdiff := :new.date1 - :new.date2;
END;
/
TRIGGER NEWTRIGGERNUM compiled

and if it stores the interval in a date, it doesn’t:

CREATE OR REPLACE TRIGGER newtriggerdate
  BEFORE INSERT ON dates FOR EACH ROW
BEGIN
  :new.datediff := :new.date1 - :new.date2;
END;
/
Error(2,11): PL/SQL: ORA-00922: missing or invalid option