如何在SQL查询中使用当前日期格式

Question

I am trying to do a query that calculates the days of past due library books in days. I also want to display the Patron's Last Name ( the person who borrowed the book), Book Title, the Due Date of the book.
I ran a simple query to understand what I am looking for. The code I ran is as follows:

    SELECT PATRON.PAT_LNAME, BOOK.BOOK_TITLE, CHECKOUT.CHECK_DUE_DATE, 
           CHECKOUT.CHECK_IN_DATE 
           FROM CHECKOUT, BOOK, PATRON
           WHERE CHECKOUT.BOOK_NUM=BOOK.BOOK_NUM 
           AND CHECKOUT.PAT_ID= PATRON.PAT_ID
           AND CHECK_IN_DATE IS NULL 

I need to calculate the days that the book is overdue from the date that it was due. I want to use the current date to calculate the days that the book is overdue, although I am not sure how to do that. I know there is a SYSDATE function that gives the current date. I do not know how to use the SYSDATE function.

Answer 1

I think the marker for a past due book is that the current date is greater than the due date. So you can just compare the due date against SYSDATE to determine which books are overdue and by how many days.

SELECT
    p.PAT_LNAME,
    b.BOOK_TITLE,
    c.CHECK_DUE_DATE, 
    c.CHECK_IN_DATE,
    SYSDATE - c.CHECK_DUE_DATE AS num_days_due
FROM CHECKOUT c
INNER JOIN BOOK b
    ON c.BOOK_NUM = b.BOOK_NUM
INNER JOIN PATRON p
    ON c.PAT_ID = p.PAT_ID
WHERE
    c.CHECK_IN_DATE IS NULL AND
    SYSDATE > c.CHECK_DUE_DATE;

In Oracle we can just subtract two dates to get the difference in days. Note also that I replaced your implicit inner joins with explicit joins. This is the preferred way of writing a join query.