Relational Database: Union Compatibility Question

Hi,

According to one reference, the union compatibility is defined as
such:
(1)the two relations have the same degree n (number of attributes) ,
and (2) domain (Ai)= domain (Bi) for 1=<i<=n, where domain stands for
data type.

In other reference, it is stated that two relations are union
compatibles if they have the same degree and the *same attribute
names* and domains. The *order of attributes is immaterial*.

which one is the fully correct?


I don't believe that the two relations should have the same attribute
names as stated in the 2nd definition , but i understand that each
pair of the two relations attributes should be of the same domain.
what about the order of attributes? Has it to be the same?

I understabd from the first definition that the attributes domains
order should be the same unlike in the second definition. which one is
correct?

The relational database is based on the concept of sets where the
order of rows and columns is not important. This tends to support
definition 2. However, if we assume that each relation has two
attributes of the same domain (eg 10 char), how the Union operator can
choose between them when mapping the attributes of relation A and B,
having different relative attributes orders


for instance Relation A schema is Fname,LName, City , Age;
the second Relation B schema is City1,FName1,Age1,LName1

where Fname,Fname1,LName,LName1 is restricted to be of 10 chars

A Union B=?


Another question, can the foreign key be NULL? are all the DBMS case
(un)sensistive?



<:> thank you <:>