sql query condition in oracle report IF ELSE - sql

Question

I have The following tables:
DOCUMENT(iddoc,doctype,title,publishingdate,validTillDate)
USERS(iduser,fname,lname)
TRANSACTION(idtrans,iduser,iddoc,transdate,schedulereturndate)
I'm asked to Indicate for a given document whether it is available or not and if it is borrowed by whom, and when it should be returned. So how can i have these conditions in my query.
where my code will be like this:
if(d.validTillDate < SYSDATE){
SELECT u.iduser t.schedulereturndate
FROM USERS u, TRANSACTION t
WHERE u.iduser=t.iduser
}
SO I WANNA KNOW HOW CAN I CODE THIS IF

Answer 1

The query for a borrowed document would be something like this:
SELECT d.iddoc,u.iduser t.schedulereturndate,'Borrowed'
from document d,
,USERS u
,TRANSACTION t
WHERE u.iduser=t.iduser
and t.iddoc=d.iddoc
and d.validitydate<sysdate
union
SELECT d.iddoc,null,null,'Not borrowed'
from document d,
WHERE d.validitydate is null
or d.validitydate>=sysdate
Edit ) added a union for the not borrowed documents.

Answer 2

It's hard to understand your question for me.
If I guessed right, then:
SELECT d.iddoc, u.iduser, t.schedulereturndate
FROM
document d
LEFT JOIN transaction t ON
(d.iddoc=t.iddoc)
-- join by iddoc field
AND (SYSDATE BETWEEN t.transdate AND t.schedulereturndate)
-- I assume we need only current transactions, not past (or future?)
WHERE
(SYSDATE<=d.validTillDate)
-- I assume we need only documents whose validity date not passed yet
Assuming there are no active transactions for iddoc=1, one active transaction for iddoc=2 and two active transactions for iddoc=3, the result will look like:
iddoc | iduser | schedulereturndate
------+--------+-------------------
1 NULL NULL
2 534 2017-09-08
3 54334 2016-03-02
3 2433 2016-07-01