Question

for example suppose we have the two relations

R1(A,B)
R2(C,D)

also these are the tables

  A B
 a1 b1 
 a2 b2

 C  D
 c1 d1
 c2 d2

will the left outer join result be the following table?

  A   B  C  D 
  a1 b1 c1 d1
  a1 b1 c2 d2
  a2 b2 c1 d1
  a2 b2 c2 d2

we have no common attributes and this is why I don’t include any NULL.

if this result is correct then it’s essentially the same with the simple join and the cross join operator
am I wrong?

Answer 1

LEFT OUTER JOIN is always done on some common column. For your example CROSS JOIN is the only one applicable – and it will give you exactly the result you provided.

Excrept (Chapter 5, Relational Algebra) from Rebeca M. Riordan: Designing Relational Database Systems, published by Microsoft Press:

An outer join returns all the records returned by an inner join, plus
all the records from either or both of the other recordsets. The
missing (“unmatched”) values will be Null.