Oracle Sql . proper Create table which can take default timestamp, condition and triggers a sequences

SequenceTrigger.sql

CREATE SEQUENCE notify_seq
 INCREMENT BY   1
 NOCACHE
 NOCYCLE;
 DROP SEQUENCE notify_seq
 


CREATE TABLE NOTIFICATIONS(	
	NOTIFY_ID NUMBER ,
	SENDER VARCHAR2(500),
	RECEIVER VARCHAR2(500),
	MESSAGE  VARCHAR2(500),
	NOTIFY_TYPE VARCHAR2(500),
	NOTIFY_TIME timestamp default SYSTIMESTAMP ,
	READ VARCHAR2(5) DEFAULT 'false' CHECK ( READ in ('true','false')) ,
	CLEAR VARCHAR2(5) DEFAULT 'false' CHECK ( CLEAR in ('true','false'))
);


SELECT * FROM NOTIFICATIONS

DROP TABLE NOTIFICATIONS

//TRIGGER THE NOTIFY SEQUENCE
CREATE OR REPLACE TRIGGER notify_trigger BEFORE INSERT ON  NOTIFICATIONS
FOR EACH ROW
declare foo number
BEGIN
	if :NOTIFICATIONS.NOTIFY_ID is null then
		select notify_seq.nextval into foo from dual
		:NOTIFICATIONS.NOTIFY_ID := foo
	end if
END;