Answer Posted / gaurav

Yes. This situation occurs in one-to-one relationships.

For example:

create table Table1
( T1_Id integer not null primary key
, T1_Data varchar(9) not null
)
create table Table2
( T2_Id integer not null primary key
, T2_Data varchar(37) not null
, foreign key (T2_Id) references Table1 (T1_Id)
)

Why would someone design two tables like this? If it's a
one-to-one relationship, couldn't you just include T2_Data
in Table1? Yes, you could, although then you would also need
to allow T2_Data to be NULL, if not every T1_Id has a
matching T2_Id—in this case it's actually a
one-to-zero-or-one relationship.

If it's a true one-to-one relationship, where every T1_Id
has a matching T2_Id, then you could combine them without
NULLs. Then the only reason why you might want separate
tables would be if T2_Data were (a) infrequently queried,
and (b) so large that it would result in many fewer physical
rows on a physical database page, which would imply poorer
performance for queries that used only T1_Data.

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