home *** CD-ROM | disk | FTP | other *** search
/ NetNews Usenet Archive 1992 #19 / NN_1992_19.iso / spool / comp / database / 6325 < prev    next >
Encoding:
Internet Message Format  |  1992-08-25  |  1.2 KB
  1. Path: sparky!uunet!haven.umd.edu!darwin.sura.net!jvnc.net!netnews.upenn.edu!saul.cis.upenn.edu!limsoon
  2. From: limsoon@saul.cis.upenn.edu (Limsoon Wong)
  3. Newsgroups: comp.databases
  4. Subject: Re: nested SQL select
  5. Message-ID: <87028@netnews.upenn.edu>
  6. Date: 26 Aug 92 01:37:03 GMT
  7. References: <1992Aug14.071343.1758@neptune.inf.ethz.ch> <l990h3INN7it@jethro.Corp.Sun.COM> <1992Aug25.161811.6054@neptune.inf.ethz.ch>
  8. Sender: news@netnews.upenn.edu
  9. Organization: University of Pennsylvania
  10. Lines: 17
  11. Nntp-Posting-Host: saul.cis.upenn.edu
  12.  
  13. In article <1992Aug25.161811.6054@neptune.inf.ethz.ch> marti@nugget.inf.ethz.ch (Robert Marti) writes:
  14. >Editorial comment:  This posting is more of an academic exercise
  15. >than of practical interest.  All I am trying to show is that any
  16. >_pure_ relational algebra expression can be translated into a
  17. >single SQL SELECT-statement -- for what it's worth ...
  18. >
  19. [stuff deleted]
  20. >
  21. >  project[...](R - S)  ---->  project[...](R) - project[...](S)
  22.  
  23. This one is not correct under the usual set-based semantics,
  24. since after the omission of some columns, previous distinct
  25. rows in R can become identified.
  26.  
  27.                                        Limsoon Wong.
  28.  
  29.  
  30.