home *** CD-ROM | disk | FTP | other *** search
/ NetNews Usenet Archive 1992 #19 / NN_1992_19.iso / spool / comp / database / theory / 408 < prev    next >
Encoding:
Internet Message Format  |  1992-08-27  |  1.7 KB
  1. Xref: sparky comp.databases.theory:408 comp.databases:6372
  2. Newsgroups: comp.databases.theory,comp.databases
  3. Path: sparky!uunet!gatech!darwin.sura.net!mojo.eng.umd.edu!hyu
  4. From: hyu@eng.umd.edu (Henry K. Yu)
  5. Subject: Different representation of entity-relation diagrams
  6. Message-ID: <1992Aug27.154114.23295@eng.umd.edu>
  7. Date: Thu, 27 Aug 92 15:41:14 GMT
  8. Organization: University of Maryland, College Park, College of Engineering
  9. Distribution: usa
  10. Lines: 32
  11.  
  12. The entity-relation diagrams that I hav been taught specify relation
  13. between entities:
  14. One-to-One:
  15.     +----------+          /----------\          +----------+
  16.     | Entity_1 | <------- | Relation | -------> | Entity_2 |
  17.     +----------+          \----------/          +----------+
  18. One-to-Many:
  19.     +----------+          /----------\          +----------+
  20.     | Entity_1 | <------- | Relation | ------>> | Entity_2 |
  21.     +----------+          \----------/          +----------+
  22. Many-to-One:
  23.     +----------+          /----------\          +----------+
  24.     | Entity_1 | <<------ | Relation | -------> | Entity_2 |
  25.     +----------+          \----------/          +----------+
  26.  
  27. Recently, another method was presented to me.  And, I don't understand how it
  28. works.  It specifies the type of relations between each entity and relation,
  29. not between entity and entity.
  30.  
  31.     +----------+   XXXX   /----------\   XXXX   +----------+
  32.     | Entity_1 | -------- | Relation | -------- | Entity_2 |
  33.     +----------+          \----------/          +----------+
  34.  
  35. Where 'XXXX' can be one of 0:0, 0:1, 1:0, 0:N, N:0, 1:1, 1:N, N:1, N:N.
  36.  
  37. Supposely, someone from MIT came up with this method.  Can someone explain
  38. the advantage of this method?  How about ways to map from the former to the
  39. latter method and vice versa?
  40.  
  41. Thanks,
  42. hyu
  43.  
  44.