What is OUTER UNION and why is it partially compatible

relational-algebra, union

Solution

`OUTER JOIN` operator exists in relational algebra even if it might not be present in most of the Query Languages.

Selecting names and marks of students of medicine from University 1 with marks above 80

Name Biology
  A   82
  B   88
  C   90

Selecting names and grades of students of technology from University 2 with grade b or above

Name Computer Science
  F    a
  G    a+
  H    b

Say this part was done internally and now we need to see the final list of students and their qualifications who can apply for my course in say Bio-informatics. An outer union will be helpful.

Table 1 OUTER UNION Table 2

Name Marks Grade
  A   82
  B   88
  C   90
  F          a
  G          a+
  H          b

A normal `UNION` cannot do this since the attribute Marks and Grade have different domains (one takes numeric values and another has character grades). However a representation like this might prove helpful in many occasions.

Problem

I am trying to understand how a `OUTER UNION` works, and why it is only partially compatible. I am aware this operation was created to take union of tuples from two relations if the relation are not type compatible (which I understand). Examples of this operation will be great!

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