How can I write the difference between sysdate - due-date on trigger - sql

Question

CREATE TABLE penalty(
member_id number(11) NOT NULL,
punishment_cost number(3),
punishment_rate number(3),
total_cost number(11)
);
CREATE TABLE member(
member_id number(11) NOT NULL PRIMARY KEY,
member_name varchar2(255) NOT NULL,
member_surname varchar2(255) NOT NULL,
gender varchar2(30) NOT NULL,
birthdate DATE,
phone varchar2(11),
mail varchar2(255) NOT NULL,
member_password varchar2(255) NOT NULL,
address varchar2(255),
status number(1),
role_id number(10) NOT NULL
);
CREATE TABLE item(
item_id varchar2(255) NOT NULL PRIMARY KEY,
member_id number(11),
publish_date DATE,
is_occupied NUMBER(1) NOT NULL,
publisher_id number(11) NOT NULL,
category_name varchar2(255) NOT NULL,
loan_id number(10)
);
CREATE TABLE current_loan(
loan_id number(10) NOT NULL PRIMARY KEY,
loan_date DATE,
due_date DATE
);
Penalty is a week entity , but I want to ask how to find the difference between the system date and the return date of an item that a member has borrowed, and fill in the member _id field in the penalty(week entity) table and the member _id, total cost fields in the penalty table. total cost = penalty_rate * days, but this trigger's run condition should run when the return date exceeds the system date.
For example
member takes a book start date - return date : today - tomorrow , but after 2 days trigger will be inserted a new row in penalty table but ı cannot do it. because sysdate-tomorrow

Answer 1

A trigger can only run when data is modified in the table. One way to approach this would be to use a view instead of a table to calculate the penalty at runtime instead of trying to store the static data. Also, your data model could use a little adjusting to make generating that info simpler. Consider this arrangement:
/* store all info about the member */
CREATE TABLE member(
member_id number(11) NOT NULL PRIMARY KEY,
member_name varchar2(255) NOT NULL,
member_surname varchar2(255) NOT NULL,
gender varchar2(30) NOT NULL,
birthdate DATE,
phone varchar2(11),
mail varchar2(255) NOT NULL,
member_password varchar2(255) NOT NULL,
address varchar2(255),
status number(1),
role_id number(10) NOT NULL
);
/* store all info about the item, including punishment rate */
CREATE TABLE item(
item_id varchar2(255) NOT NULL PRIMARY KEY,
publish_date DATE,
publisher_id number(11) NOT NULL,
category_name varchar2(255) NOT NULL,
punishment_rate number(3)
);
/* a loan is a relationship between member and item, with dates */
CREATE TABLE current_loan(
item_id number(11) NOT NULL,
member_id number(11) NOT NULL
loan_date DATE NOT NULL,
due_date DATE NOT NULL
);
/* calculate overdue fees for each member as of current sysdate */
CREATE OR REPLACE VIEW punishment AS
SELECT
m.member_name,
sum(i.punishment_rate*trunc(sysdate-l.due_date)) total_cost
FROM
member m
left join current_loan l on (l.member_id = m.member_id)
left join item i on (i.item_id = l.item_id)
WHERE
(sysdate - l.due_date) >= 1
GROUP BY m.member_name;
The view will show the total penalties for each member across all loans, based on the difference between current sysdate and due date, only for overdue returns. Because it is a view, the total amount is calculated at runtime and is always current. Alternatively, if you must store the actual total in a column, you could implement this as a materialized view that would refresh on a set schedule. This approach would only be recommended if you were performing so many calculations when querying the view that it affected performance of the query.
Note: You include a member_password column in your member table. not sure what you actually plan to store there, but storing the actual password for a user account is always a bad idea. At most you should only store a one way hash of the password, probably as a RAW data type.