Tech or Treat! Write an article about your scariest tech disaster to win gadgets!Learn more

x
?
Solved

Entity relation

Posted on 1998-09-24
1
Medium Priority
?
599 Views
Last Modified: 2008-03-06
Are weak entities necessary?  What is the distinction between a weak entity and a strong one?  Can a weak entity be converted to a strong entity?
0
Comment
Question by:hemag
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
1 Comment
 
LVL 3

Accepted Solution

by:
junfeb earned 30 total points
ID: 1025749

A weak entity (``W'') is a component of an entity-relationship diagram that has all the characteristics of an entity, except that there is no set of attributes of the
weak entity that forms a key, and such that there is an entity (or another weak entity) E, such that

     every instance of W is associated with exactly one instance of E,
     a ``primary key'' for the weak entity W can be obtained from the primary key for E by including one or more of the attributes of W.

Thus the primary key of a weak entity is not a subset of (just) the set of the weak entity's attributes; it includes attributes from another entity as well (or, maybe
even several other entities, if this weak entity is associated to another weak entity).

Note, by the way, that this definition does not imply that each instance of the ``strong'' entity E must have an instance of W associated to it. In fact, an instance
of E could have zero, one, or even many instances of W associated to it.

 you should use a weak entity when you would use a (regular) entity, but you find that a key can't be formed from the available attributes, and when each instance of the new weak entity is associated with - or depends on - exactly one instance of some entity.

0

Featured Post

Concerto Cloud for Software Providers & ISVs

Can Concerto Cloud Services help you focus on evolving your application offerings, while delivering the best cloud experience to your customers? From DevOps to revenue models and customer support, the answer is yes!

Learn how Concerto can help you.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

In this blog post, we’ll look at how ClickHouse performs in a general analytical workload using the star schema benchmark test.
In this blog post, we’ll look at how using thread_statistics can cause high memory usage.
This is a high-level webinar that covers the history of enterprise open source database use. It addresses both the advantages companies see in using open source database technologies, as well as the fears and reservations they might have. In this…
In this video, Percona Director of Solution Engineering Jon Tobin discusses the function and features of Percona Server for MongoDB. How Percona can help Percona can help you determine if Percona Server for MongoDB is the right solution for …
Suggested Courses

647 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question