how to add a database constraint 18 or above - sql

Question

i have create a table and I need to add a database constraint "A customer with an age under 18 cannot rent a movie rated “18 or above”." How do I use SQL to add this in my table

Answer 1

You can use a trigger. When you want to enter information in the system to order a movie, do not allow the user to create a record if the age is less than the minimum required.
The following code snippet implements this scenario completely.
CREATE TABLE Customers
(
ID int identity,
FirstName nvarchar(100),
LastName nvarchar(100),
Age int,
primary key(ID))
CREATE TABLE Films
(
ID int identity,
FilmName nvarchar(100),
MinimumAge int,
primary key(ID))
CREATE TABLE OrderFilms
(
ID int identity,
CustomerID int,
FilmID int,
primary key(ID),
foreign key(CustomerID) references Customers,
foreign key(FilmID) references Films)
create trigger trigger_for_insert_into_OrderFilms
On OrderFilms
For Insert
AS
Declare #film int
Declare #customer int
Declare #filmage int
Declare #customerage int
set #film = (SELECT top 1 FilmID FROM inserted)
set #customer = (SELECT top 1 CustomerID FROM inserted)
set #filmage = (SELECT MinimumAge FROM Films WHERE ID = #film)
set #customerage = (SELECT Age FROM Customers WHERE ID = #customer)
IF(#filmage > 18 AND #customerage < 18)
BEGIN
PRINt 'ERROR, CUSTOMER AGE THE CUSTOMER IS YOUNG'
RollBack
END