CREATE FUNCTION for attribute to at most 3 people? - sql

Question

I am currently doing a hotel booking application on SQL Server 2018, and am trying to write a constraint for the RoomNo attribute of my SQL Server table. Essentially, I want each RoomNo to only be able to have at most 3 person, but ran into an error when trying to do the CREATE FUNCTION.
This are my current code:
CREATE TABLE Passenger
(
ID smallint ,
Name varchar (50) NOT NULL,
Email varchar (319) NULL,
DOB smalldatetime NOT NULL,
Gender char (1) NOT NULL CHECK (Gender IN ('M', 'F')),
RoomNo tinyint NOT NULL,
CONSTRAINT PK_Passenger PRIMARY KEY NONCLUSTERED (ID),
CONSTRAINT CHK_Passenger_Gender CHECK (Gender IN ('M', 'F'))
)
CREATE FUNCTION CalculateRoomNo
(
#value tinyint
)
RETURNS bit
AS
BEGIN
IF (SELECT COUNT(RoomNo) FROM Passenger GROUP BY RoomNo) <= 3
RETURN 0
RETURN 1
END
GO
ALTER TABLE Passenger
ADD CONSTRAINT CHK_RoomNoPax CHECK (dbo.CalculateRoomNo(RoomNo) = 0)
GO
When I add a passenger into the table, if it is formatted like this:
INSERT INTO Passenger
VALUES (1, 'Rob', 'Rob#gmail.com', '2017-10-04', 'M', 12)
INSERT INTO Passenger
VALUES (2, 'Darren', 'Darren#yahoo,com', '1976-12-21', 'F', 12)
INSERT INTO Passenger
VALUES (3, 'Peggy', '', '2006-03-15', 'F', 12)
INSERT INTO Passenger
VALUES (4, 'Carlos', '', '1981-04-06', 'F', 12)
It will stop at
INSERT INTO Passenger VALUES (3, 'Peggy', '', '2006-03-15', 'F', 12)
since RoomNo '12' has reached its maximum capacity.
But, if I added the values like such where the room numbers are different from each other:
INSERT INTO Passenger
VALUES (1, 'Rob', 'Rob#gmail.com', '2017-10-04', 'M', 69)
INSERT INTO Passenger
VALUES (2, 'Darren', 'Darren#yahoo,com', '1976-12-21', 'F', 74)
INSERT INTO Passenger
VALUES (3, 'Peggy', '', '2006-03-15', 'F', 45)
INSERT INTO Passenger
VALUES (4, 'Carlos', '', '1981-04-06', 'F', 72)
INSERT INTO Passenger
VALUES (5, 'John', 'johnny#hotmail.com', '1988-05-06', 'M', 69)
It will return an error:
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
Is there any way I can properly run this SQL?

Answer 1

The query with the GROUP BY can return more than 1 record if there's more than 1 RoomNo.
If you include a WHERE clause for the RoomNo then it can only be 1 COUNT
CREATE FUNCTION CalculateRoomNo
(
#RoomNo tinyint
)
RETURNS bit
AS
BEGIN
IF (SELECT COUNT(*) FROM Passenger WHERE RoomNo = #RoomNo) <= 3
RETURN 0
RETURN 1
END
Demo on db<>fiddle here

Answer 2

As mentioned by #LukStorms, your query has no filter on RoomNo, therefore it can return multiple rows. A scalar subquery must return a maximum of one row.
But the most correct way to achieve what you are trying to do, is not to use this function at all. Instead you can add another column, and create a unique constraint across that and the RoomNo
ALTER TABLE Passenger
ADD RoomNoPax tinyint NOT NULL
CONSTRAINT CHK_RoomNoPax CHECK (RoomNoPax >= 1 AND RoomNoPax <= 3);
ALTER TABLE Passenger
ADD CONSTRAINT UQ_RoomNo_RoomNoPax UNIQUE (RoomNo, RoomNoPax);
db<>fiddle
You now have an extra column which must have the value 1, 2 or 3. And there is a unique constraint over every pair of that value and the RoomNo, so you cannot now put more than 3 Passenger in each RoomNo.

Answer 3

You need to change the logic in the function.
You should be checking for the number of records for any given roomno, not the number of roomno in the entire table.
the subquery should return only one scalar value because you are performing a logical operation.
i.e.
if exists (select 1 from Passenger group by roomno having count(1) <= 3)
begin
return 1
end
else
begin
return 0
end
In the above query, we are checking for the number of persons assigned to each room number and if there is an existence of such case then it will return 1. In this case, it will not return more than one record.
Please modify the return value as per your requirement.
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