SQL library trigger - sql

Question

Im trying to make a library database. I would like to ensure that one book can be borrowed just to one person at a time. I have no expirience with triggers so I thought I might ask you.
create table "book" (
"book_id" INTEGER not null,
"condition" VARCHAR2(50),
"isbn" VARCHAR2(50) not null,
constraint PK_BOOK primary key ("book_id")
);
create table "borrowed" (
"book_id" INTEGER not null,
"borrowed_id" INTEGER not null,
"user_id" INTEGER not null,
"date_borrowing" DATE not null,
"date_returning" DATE not null,
"returned" SMALLINT not null,
constraint PK_BORROWED primary key ("book_id", "borrowed_id")
);
atribute "returned" has just yes or no value (1 or 0)

Answer 1

You don't need a trigger for this. Your borrowed table should be structured like this. Note the use of a virtual column, which requires Oracle 11g+:
create table borrowed (
borrowed_id INTEGER not null primary key, -- this should be set to a unique id for each row, using your preferred method
book_id INTEGER not null,
user_id INTEGER not null,
date_borrowing DATE not null,
date_returning DATE not null,
date_actualreturn DATE,
returned as (case date_actualreturn is null then 0 else 1 end)
);
Then, you want date_actualreturn to have at most one NULL value per book. You can do this with a unique index or constraint:
create table borrowed (
borrowed_id INTEGER not null primary key, -- this should be set to a unique id for each row, using your preferred method
book_id INTEGER not null,
user_id INTEGER not null,
date_borrowing DATE not null,
date_returning DATE not null,
date_actualreturn DATE,
returned as (case date_actualreturn is null then 0 else 1 end),
constraint unq_only_one_book_borrowed unique
(coalesce(date_actualreturn, date '1900-01-01'), book_id)
);
This creates a unique constraint on the column. If a book has not been returned, the date always looks the same -- so the same book cannot be borrowed twice. And, voila! No trigger.