I want to create AFTER UPDATE trigger to update DATE OF BIRTH where (AGE < 20) - sql

Question

I want to update the data in table where age is less than 20. So I want to update dob and set it to date where age is greater than 20 (by using after update trigger). I know what I'm saying doesn't make any sense but this is the task given by my proffesor.
and I'm using Oracle Live SQL.
create table SYCS_DBMS(
SID NUMBER(10),
SNAME VARCHAR(50),
DOB DATE,
PRESENT DATE
);
ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YYYY';
INSERT INTO SYCS_DBMS VALUES(1, 'ANKIT', '4-OCTOBER-2002', sysdate);
INSERT INTO SYCS_DBMS VALUES(2, 'AAKASH', '30-OCTOBER-2002', sysdate);
INSERT INTO SYCS_DBMS VALUES(3, 'DHRUV', '5-APRIL-2002', sysdate);
INSERT INTO SYCS_DBMS VALUES(4, 'DEERAJ', '5-AUGUST-2002', sysdate);
INSERT INTO SYCS_DBMS VALUES(5, 'ADIL', '11-MARCH-2003', sysdate);
INSERT INTO SYCS_DBMS VALUES(6, 'VIRAJ', '7-JULY-2002', sysdate);
CREATE OR REPLACE TRIGGER trUPDT
AFTER UPDATE
ON SYCS_DBMS
FOR EACH ROW
DECLARE
STU_AGE NUMBER;
BEGIN
--TO CHECK THE AGE BY DATE OF BIRTH
SELECT MONTHS_BETWEEN(TO_DATE(sysdate,'DD-MON-YYYY'), TO_DATE(:NEW.DOB,'DD-MON-YYYY'))/12
INTO STU_AGE FROM DUAL;
END;

Answer 1

Let's take a walk down "Revision Lane" as this code can make use of some. But first as work about formatting. You need to learn how to do it. In fact I tell my students that if they submit unformatted code they fail that assignment. I don not even bother to read it. Format your code. I should be able to get an idea of the flow just looking at it. It does not really matter how you format, just as long as it consistent through out. Admittedly it is not a big deal for 10 lines of code, but the time to form good habits is before forming bad ones. Now for what you have:
Be careful setting and depending on setting NLS_DATE_FORMAT for here
it is fine. But what happens when you need to call another routine
an that developer also sets NLS_DATE_FORMAT but different from yours.
Once a column/parameter is a date data type never use the to_date function on it, it is already a date. Oracle provides a slew of date processing functions and makes date arithmetic available. In this case both sysdate and the column DOB are already dates so do not do to_date() on them.
Your expression
months_between(to_date(sysdate,'dd-mon-yyyy'),
to_date(:new.dob,'dd-mon-yyyy'))/12
Becomes just simply
months_between(sysdate, :new.dob)/12
Going a step further. Within a plsql block there is no need to Select ... into unless you are actually retrieving from a table (or view, etc). You can just make a direct assignment to a variable.
So with this and the above:
select months_between(to_date(sysdate, :new.dob)/12
into stu_age
from dual;
becomes a simple assignment:
stu_age := months_between(to_date(sysdate, :new.dob)/12;
At this point your trigger has been reduced to:
create or replace trigger trupdt
after update
on sycs_dbms
for each row
declare
stu_age number;
begin
--to check the age by date of birth
stu_age := months_between(sysdate,:new.dob)/12;
end trupdt;
Unfortunately, it is completely useless. You computed the months between the 2 dates as a numeric value (with decimal places), but then the trigger ends and just throws that calculation away doing nothing with it. I do not think this is what you after. You initially stated you wanted to update dob. Well there are 2 issues here.
DOB is defined as a date so your calculation needs to result in a date. This does not.
You have a after update trigger but an after update trigger
cannot change column values, You need a before update trigger.
Now you could the calculated value and a date calculation to get the DOB value, with the appropriate logic to determine if you even should. But the suggestion by #Barmar is extremely good. I'll us it. Thanks Barmer. Instead of the months_between function use the add_months function with sysdate to calculate the date 20 years ago, then use the least function to choose the appropriate value. Also make a direct assignment to :new_dob bypassing the local variable. The final trigger for this becomes simply:
create or replace trigger sycs_dbms_dob_ge20_bur
before update
on sycs_dbms
for each row
begin
:new.dob := least(:new.dob, add_months(sysdate, -240)); -- (20yrs * 12mon/yr)
end sycs_dbms_dob_ge20_bur;
I have put together a fiddle here that walks through each step above. I would not typically provide a complete answer to an obvious homework assignment. But if I had assigned this I would have walked through it in the next class. My hope is you take this to class and discuss it the professor and other students. Do not just submit as your own, any half-way decent professor would catch on real fast. But at least study carefully what it does and how it got there from where you started.