What is OUTER UNION and why is it partially compatible

What is OUTER UNION and why is it partially compatible

References

Reference NO 1

http://stackoverflow.com/questions/13588291/what-is-outer-union-and-why-is-it-partially-compatible

Reference NO 2

http://cs.stackexchange.com/questions/6997/what-is-outer-union-and-why-is-it-partially-compatible

Reference NO 3

http://cisnet.baruch.cuny.edu/holowczak/classes/3400/relationalalgebra/

Reference NO 4

http://120.105.96.8/lab/past_course/96-1/database/hw5-answer.doc

Reference NO 5

http://www.cs.gsu.edu/~cscbecx/3360%20DateBase%20Fall%202008/Chapter%206%20additional.ppt

Reference NO 6

http://cs.ulb.ac.be/public/_media/teaching/infoh303/algnotes2p.pdf

Reference NO 7

http://home.adelphi.edu/~siegfried/cs443/443l6.pdf

Reference NO 8

http://www.cs.gmu.edu/~huangyih/450/Lec06-algebra2.ppt