create oracle DB auto increment PK trigger

DROP SEQUENCE seq_chat_group_Id;
DROP SEQUENCE seq_chat_msg_id;

CREATE SEQUENCE seq_chat_group_Id;
CREATE SEQUENCE seq_chat_msg_id;


CREATE OR REPLACE TRIGGER autoIncrementMsgPK_trigger
BEFORE INSERT ON chat_messages
FOR EACH ROW WHEN (new.msgId IS NULL)
BEGIN
    SELECT seq_chat_msg_id.nextval INTO:new.msgId FROM dual;
END;

CREATE OR REPLACE TRIGGER autoIncrementGroupPK_trigger
BEFORE INSERT ON chat_group
FOR EACH ROW WHEN (new.groupId IS NULL)
BEGIN
    SELECT seq_chat_group_Id.nextval INTO:new.groupId FROM dual;
END;