Statement level trigger to enforce a constraint - oracle

Question

I am trying to implement a statement level trigger to enforce the following "An applicant cannot apply for more than two positions in one day".
I am able to enforce it using a row level trigger (as shown below) but I have no clue how to do so using a statement level trigger when I can't use :NEW or :OLD.
I know there are alternatives to using a trigger but I am revising for my exam that would have a similar question so I would appreciate any help.
CREATE TABLE APPLIES(
anumber NUMBER(6) NOT NULL, /* applicant number */
pnumber NUMBER(8) NOT NULL, /* position number */
appDate DATE NOT NULL, /* application date*/
CONSTRAINT APPLIES_pkey PRIMARY KEY(anumber, pnumber)
);
CREATE OR REPLACE TRIGGER app_trigger
BEFORE INSERT ON APPLIES
FOR EACH ROW
DECLARE
counter NUMBER;
BEGIN
SELECT COUNT(*) INTO counter
FROM APPLIES
WHERE anumber = :NEW.anumber
AND to_char(appDate, 'DD-MON-YYYY') = to_char(:NEW.appDate, 'DD-MON-YYYY');
IF counter = 2 THEN
RAISE_APPLICATION_ERROR(-20001, 'error msg');
END IF;
END;

Answer 1

You're correct that you don't have :OLD and :NEW values - so you need to check the entire table to see if the condition (let's not call it a "constraint", as that term has specific meaning in the sense of a relational database) has been violated:
CREATE OR REPLACE TRIGGER APPLIES_AIU
AFTER INSERT OR UPDATE ON APPLIES
BEGIN
FOR aRow IN (SELECT ANUMBER,
TRUNC(APPDATE) AS APPDATE,
COUNT(*) AS APPLICATION_COUNT
FROM APPLIES
GROUP BY ANUMBER, TRUNC(APPDATE)
HAVING COUNT(*) > 2)
LOOP
-- If we get to here it means we have at least one user who has applied
-- for more than two jobs in a single day.
RAISE_APPLICATION_ERROR(-20002, 'Applicant ' || aRow.ANUMBER ||
' applied for ' || aRow.APPLICATION_COUNT ||
' jobs on ' ||
TO_CHAR(aRow.APPDATE, 'DD-MON-YYYY'));
END LOOP;
END APPLIES_AIU;
It's a good idea to add an index to support this query so it will run efficiently:
CREATE INDEX APPLIES_BIU_INDEX
ON APPLIES(ANUMBER, TRUNC(APPDATE));
dbfiddle here
Best of luck.

Answer 2

Your rule involves more than one row at the same time. So you cannot use a FOR ROW LEVEL trigger: querying on APPLIES as you propose would hurl ORA-04091: table is mutating exception.
So, AFTER statement it is.
CREATE OR REPLACE TRIGGER app_trigger
AFTER INSERT OR UPDATE ON APPLIES
DECLARE
cursor c_cnt is
SELECT 1 INTO counter
FROM APPLIES
group by anumber, trunc(appDate) having count(*) > 2;
dummy number;
BEGIN
open c_cnt;
fetch c_cnt in dummy;
if c_cnt%found then
close c_cnt;
RAISE_APPLICATION_ERROR(-20001, 'error msg');
end if;
close c_cnt;
END;
Obviously, querying the whole table will be inefficient at scale. (One of the reasons why triggers are not recommended for this sort of thing). So this is a situation in which we might want to use a compound trigger (assuming we're on 11g or later).