foreign key

at what situation foreign key's value can have null value?
and in what situation foreign key can't have null value?

foreign key can have a null value if the design of the application allows it

foreign key can be declared not null if every row in the child table must have a parent row

you want examples?

create table people
( id integer not null primary key
, fullname varchar(50) not null
, married_to integer null foreign key references people(id)
)

being married is optional, so the FK may be null

create table cities
( id integer not null primary key
, cityname varchar(100) not null
, country_code char(2) not null foreign key references country(code)
)

a city must be in some country, so that FK is not null


rudy
http://r937.com/

thx... for the explanation...