SQL Server trigger only on changes to update a calculated column - sql-server

Question

For a time scheduling project, I have two tables: tbl_timeslots which holds available times with slotid as the primary key and totalmembers which counts the number of appointments made for this slot, and tbl_appointments with primary key apptid which holds the actual appointments, with slotid as a foreign key linking to the slot information.
I need to automatically update the totalmembers column any time an appointment is created/deleted/changed. The trigger I wrote (shown below) does not update the correct number of appointments in the tbl_timeslots column totalmembers.
CREATE TABLE tbl_timeslots
(
slotid int ,
fromdate datetime ,
todate datetime ,
totalmembers int
)
INSERT tbl_timeslots (slotid, fromdate, todate, totalmembers)
VALUES (1, '2016-01-01 10:00:00', '2016-01-01 11:00:00', 0)
INSERT tbl_timeslots (slotid, fromdate, todate, totalmembers)
VALUES (2, '2016-01-01 11:00:00', '2016-01-01 12:00:00', 0)
CREATE TABLE tbl_appointments
(
apptid int ,
slotid int ,
firstname varchar(10) ,
lastname varchar(10)
)
INSERT tbl_appointments (apptid, slotid, firstname, lastname)
VALUES (1, 1, 'Mark', 'Twain')
INSERT tbl_appointments (apptid, slotid, firstname, lastname)
VALUES (2, 1, 'Thomas', 'Jefferson')
INSERT tbl_appointments (apptid, slotid, firstname, lastname)
VALUES (3, 2, 'Donald', 'Duck')
CREATE TRIGGER [dbo].[tr_totalmembers]
ON [dbo].[TBL_appointments]
AFTER UPDATE, INSERT, DELETE
AS
BEGIN
UPDATE tbl_timeslots
SET totalmembers = (SELECT COUNT(1)
FROM tbl_appointments a
WHERE tbl_timeslots.slotid = a.slotid)
FROM inserted i
INNER JOIN deleted d ON i.apptid = d.apptid
WHERE
d.slotid <> i.slotid
AND (tbl_timeslots.slotid = i.slotid OR tbl_timeslots.slotid = d.slotid)
END

Answer 1

I just modified the trigger a bit and it worked for me :
CREATE TRIGGER [dbo].[tr_totalmembers]
ON [dbo].[TBL_appointments]
AFTER UPDATE, INSERT, DELETE
AS
BEGIN
UPDATE tbl_timeslots
SET totalmembers = a.cnt
from
(
SELECT slotid,COUNT(1) as cnt
FROM tbl_appointments
group by slotid
) a
WHERE
tbl_timeslots.slotid=a.slotid
END
Or in case if there is a specific problem please mention so that we could look into it .

Answer 2

A better way to compute totalmembers is to create and use an indexed view:
CREATE VIEW dbo.vw_timeslots_with_totalmembers
WITH SCHEMA_BINDING
AS
SELECT a.slotid, COUNT_BIG(*) AS totalmembers
FROM dbo.tbl_appointments a
GROUP BY a.slotid
GO
CREATE UNIQUE CLUSTERED INDEX IUC_vw_timeslots_with_totalmembers_slotid
ON dbo.vw_timeslots_with_totalmembers (slotid)
GO
DECLARE #slotid INT = 123
SELECT totalmembers
FROM dbo.vw_timeslots_with_totalmembers WITH(NOEXPAND) -- This table hint is needed in order to force usage of indexed view
WHERE slotid = #slotid
GO
Note: please read following notes regarding the proper configuration of SETtings (see section Required SET Options for Indexed Views): https://msdn.microsoft.com/en-us/library/ms191432.aspx .
Note #2: if last SELECT statement returns 0 rows this means that current slot doesn't have appointments (I assume that current slot is valid).