What is OUTER UNION and why is it partially compatible

Question

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!

Cerement

anyone came across relational algebra operator/symbol to represent outer union? for example full outer join is represented with ⟗. Is something similar for outer union already there? – Liquidate

Answer 1

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.

Fearless

Answer 2

Where have you heard of the existence of such an operator ?

It does not exist in standard SQL afaict, and a google search seems to lead only to syntax for PROC SQL in SAS.

At any rate, if the output of this operator is a table that possibly contains NULLs, then this operator is not part of the relational algebra. Relational algebra is computation on relations that produce other relations in turn, and something that contains NULLs is not a relation.

Gonzalo

he is talking about relational algebra – Eer