Constraint ?

Question

what is constraint , I found that is the rule for the data type of the table, but i'm still confused , when I have to use it ?

for example :

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    CONSTRAINT FK_PersonOrder FOREIGNKEY (PersonID)
    REFERENCES Persons(PersonID)
);

Answer 1

It says that in order to INSERT a line, an entry with the same PersonID must be in table Persons.

Answer 2

OK but why I have to put the constraint key , why I can't do just this ..... FK_PersonOrder FOREIGNKEY (PersonID) REFERENCES Persons(PersonID) ?

Answer 3

Because you have to start with some keyword. Fk_PersonOrder is the name for the foreign key.

If the CONSTRAINT wasn't there the line would start with the name.

Answer 4

AAA I get it now , thank you ,
it was really helpful , because I am going to pass an exam on it