Question

I have a table called ward (dealing with hospitals):

Create table ward(
    Wno varchar(15) Primary Key,
    Name varchar(20) Not Null,
    Number_of_beds integer Not Null
);

And a table for patients:

Create table patient(
    Pid varchar(15) Primary Key,
    Name varchar(20) Not Null,
    Address varchar(50) Not Null,
    Date_of_birth date Not Null
 );

I need to constrain the patients somehow so that if a patient is assigned to a specific ward then the number of patients can’t exceed the number of beds in the ward.
I thought of adding the Wno as a foreign key to the patient table, but don’t really know where to go from there.

Answer 1

You may add the foreign key into patient table as following. Apology this query is in MYSQL. I noticed that you need Oracle though. However the logic is similar 🙂 The syntax needs changes.

Create table ward(
    Wno varchar(15) Not null Primary Key,
    Name varchar(20) Not Null,
    Number_of_beds integer Not Null
);

And a table for patients:

Create table patient(
    Pid varchar(15) Primary Key,
    Name varchar(20) Not Null,
    Address varchar(50) Not Null,
    Date_of_birth date Not Null,
    WardNo varchar(15),
    foreign key (wardno) references ward (wno)  ' -- adds the foreign key relation
 );

In order to check if ward is full, you can have an insert or update trigger

Free bed count can be obtained by following query:

SELECT p.wardno, (w.number_of_beds - count(pid)) as freebeds
from patient as p
left join ward as w
on p.wardno = w.wno
group by wardno

Now we create a trigger to check if any patient is going into a ward whre freebed count = 0.

Updated to Oracle Version

 CREATE OR REPLACE TRIGGER FreeBedsWardTrigger
       BEFORE UPDATE OR INSERT
       ON patient
       FOR EACH ROW
    DECLARE
       max_beds    INTEGER;    -- max number of beds for the ward
       used_beds   INTEGER;    -- used beds for the ward
    BEGIN
       SELECT   COUNT (pid)
         INTO   used_beds
         FROM   patient
        WHERE   wardno = :NEW.wardno;

       SELECT   number_of_beds
         INTO   max_beds
         FROM   ward
        WHERE   wno = :NEW.wardno;

       IF (max_beds - used_beds) > 0
       THEN
          RETURN;
       ELSE
          RAISE_APPLICATION_ERROR (-100100,
                                   'No more beds available in this ward.');
       END IF;
    END;