How can I create this trigger for Microsoft SQL Server? - sql-server

Question

I have this limitation I have to implement server-side (in the database). Normally I'd do this on the client side, but I'd figure why not learn the server aspect. :p
I don't want my Clients to be able to rent a Movie from the XXX genre if they are under 18.
Here is the script I used to generate the tables:
-- =============================================
-- Sergio's Lab Tests MWA HA HA
-- =============================================
use AlquilerPeliculas
create table Client
(
ID int primary key not null identity(1,1),
Address nvarchar(1024) not null,
Phone nvarchar(256) not null,
NIT nvarchar(32) not null
)
go
create table Genre
(
ID int primary key not null identity(1,1),
Name nvarchar(256)
)
go
create table Movie
(
ID int primary key not null identity(1,1),
Name nvarchar(256) not null,
IDGenre int foreign key references Genre(ID)
)
go
create table Natural
(
IDCliente int primary key references Cliente(ID),
Age as datediff(d, FechaDeNacimiento,getdate())/365.00,
Nombre nvarchar(1024) not null,
ApellidoPaterno nvarchar(512) not null,
FechaDeNacimiento datetime,
Sexo varchar(1) not null check(Sexo='M' or Sexo='F')
)
go
create table Alquiler
(
ID int primary key not null identity(1,1),
FechaDeAlquiler datetime,
Total nvarchar(20) not null,
IDClient int foreign key references Client(ID)
)
go
create table Ejemplar
(
ID int primary key not null identity(1,1),
NumeroDeEjemplar nvarchar(256) not null,
Descripcion nvarchar(1024),
IDFormato int foreign key references Formato(ID),
IDPelicula int foreign key references Pelicula(ID)
)
go
create table DetalleAlquiler
(
ID int primary key not null identity(1,1),
IDEjemplar int foreign key references Ejemplar(ID),
IDAlquiler int foreign key references Alquiler(ID),
PrecioDeAlquiler nvarchar(128),
FechaDevolucion datetime,
FechaDevolucionProgramada datetime
)
I asked a friend what I should use and he said a Trigger, but it's my understanding that a trigger is a function that's run when the triggers conditions are met, right? If I used a trigger I'd have to insert then delete a naughty record right?
Thanks for the help.

Answer 1

Read all about triggers here
They come in many flavors, they can run before every insert / after every update and so on. The triggering of the trigger is a blanket action, if you have a BEFORE trigger, it will always run before.
Then, if you want to do any filtering or error handling, for example, only run a bit of code if a particular column has a particular value, you include that inside the trigger (in conditional code).
Generally, I recommend against using triggers, they can often make locking more complicated and are "hidden" in a place no one tends to look (which make them very hard to debug).
Another approach you can take DB-wise, it have particular stored procs that you call, instead of calling tables directly, eg: spRentMovie.