Constrain number of rows in table, by value in another table - sql

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;