SQL - Trigger for auto-incrementing number of signed in people - sql-server

Question

I am having a little bit of trouble with making a trigger in my SQL. I have two tables:
This one
Create table [user]
(
[id_user] Integer Identity(1,1) NOT NULL,
[id_event] Integer NULL,
[name] Nvarchar(15) NOT NULL,
[lastname] Nvarchar(25) NOT NULL,
[email] Nvarchar(50) NOT NULL, UNIQUE ([email]),
[phone] Integer NULL, UNIQUE ([phone]),
[pass] Nvarchar(50) NOT NULL,
[nick] Nvarchar(20) NOT NULL, UNIQUE ([nick]),
Primary Key ([id_user])
)
go
and this one
Create table [event]
(
[id_event] Integer Identity(1,1) NOT NULL,
[id_creator] Integer NOT NULL,
[name] Nvarchar(50) NOT NULL,
[date] Datetime NOT NULL, UNIQUE ([date]),
[city] Nvarchar(50) NOT NULL,
[street] Nvarchar(50) NOT NULL,
[zip] Integer NOT NULL,
[building_number] Integer NOT NULL,
[n_signed_people] Integer Default 0 NOT NULL Constraint [n_signed_people] Check (n_signed_people <= 20),
Primary Key ([id_akce])
)
Now I need a trigger for when I insert a new user with and id_event, or update existing one with one, to take the id_event I inserted, look in the table of events and increment the n_signed_people in a line with a coresponding id_event, until it is 20. When it is 20, it should say that the event is full. I made something like this, it is working when I add a new user with id, but now I need it to stop at 20 and say its full and also I am not sure if it will work, when I'll try to update existing user, by adding an id_event (I assume it was NULL before update).
CREATE TRIGGER TR_userSigning
ON user
AFTER INSERT
AS
BEGIN
DECLARE #idevent int;
IF (SELECT id_event FROM Inserted) IS NOT NULL --if the id_event is not empty
BEGIN
SELECT #idevent=id_event FROM Inserted; --the inserted id_event will be save in a local variable
UPDATE event SET n_signed_people = n_signed_people+1 WHERE #idevent = id_event;
END
END
go

Answer 1

Good evening,
I did notice some issues with your schema. I want to list the fixes I made in order.
1 - Do not use reserved words. Both user and event are reserved.
2 - Name your constraints. You will be glad they are not some random word when you want to drop one.
3 - I added a foreign key to make sure there is integrity in the relationship.
All this work was done in tempdb. Now, lets get to the fun stuff, the trigger.
-- Just playing around
use tempdb;
go
-- attendee table
if object_id('attendees') > 0
drop table attendees
go
create table attendees
(
id int identity (1,1) NOT NULL constraint pk_attendees primary key,
firstname nvarchar(15) NOT NULL,
lastname nvarchar(25) NOT NULL,
email nvarchar(50) NOT NULL constraint uc_email unique,
phone int NULL constraint uc_phone unique,
pass nvarchar(50) NOT NULL,
nick nvarchar(20) NOT NULL constraint uc_nick unique,
event_id int NOT NULL
)
go
-- events table
if object_id('events') > 0
drop table events
go
create table events
(
id int identity (1,1) NOT NULL constraint pk_events primary key,
creator int NOT NULL,
name nvarchar(50) NOT NULL,
planed_date datetime NOT NULL constraint uc_planed_date unique,
street nvarchar(50) NOT NULL,
city nvarchar(50) NOT NULL,
zip nvarchar(9) NOT NULL,
building_num int NOT NULL,
registered int
constraint df_registered default (0) NOT NULL
constraint chk_registered check (registered <= 20),
);
go
-- add some data
insert into events (creator, name, planed_date, street, city, zip, building_num)
values (1, 'new years eve', '20131231 20:00:00', 'Promenade Street', 'Providence', '02908', 99);
-- make sure their is integrity
alter table attendees add constraint [fk_event_id]
foreign key (event_id) references events (id);
I usually add all three options (insert, update, & delete). You coded for insert in the example above. But you did not code for delete.
Also, both the inserted and deleted tables can contain multiple rows. For instance, if two attendees decide to drop out, you want to minus 2 from the table.
-- create the new trigger.
CREATE TRIGGER [dbo].[trg_attendees_cnt] on [dbo].[attendees]
FOR INSERT, UPDATE, DELETE
AS
BEGIN
-- declare local variable
DECLARE #MYMSG VARCHAR(250);
-- nothing to do?
IF (##rowcount = 0) RETURN;
-- do not count rows
SET NOCOUNT ON;
-- deleted data
IF NOT EXISTS (SELECT * FROM inserted)
BEGIN
UPDATE e
SET e.registered = e.registered - c.total
FROM
[dbo].[events] e
INNER JOIN
(SELECT [event_id], count(*) as total
FROM deleted group by [event_id]) c
ON e.id = c.event_id;
RETURN;
END
-- inserted data
ELSE IF NOT EXISTS (SELECT * FROM deleted)
BEGIN
UPDATE e
SET e.registered = e.registered + c.total
FROM
[dbo].[events] e
INNER JOIN
(SELECT [event_id], count(*) as total
FROM inserted group by [event_id]) c
ON e.id = c.event_id;
RETURN;
END;
-- updated data (no counting involved)
END
GO
Like any good programmer, I need to test my work to make sure it is sound.
Lets add 21 new attendees. The check constraint should fire. This only works since the error generated by the UPDATE rollback the insert.
-- Add 21 attendees
declare #var_cnt int = 0;
declare #var_num char(2);
while (#var_cnt < 22)
begin
set #var_num = str(#var_cnt, 2, 0);
insert into attendees (firstname, lastname, email, phone, pass, nick, event_id)
values ('first-' + #var_num,
'last-' + #var_num,
'email-'+ #var_num,
5554400 + (#var_cnt),
'pass-' + #var_num,
'nick-' + #var_num, 1);
set #var_cnt = #var_cnt + 1
end
go
Last but not least, we need to test a DELETE action.
-- Delete the last row
delete from [dbo].[attendees] where id = 20;
go