Solved

Entity relation

Posted on 1998-09-24
1
561 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
1 Comment
 
LVL 3

Accepted Solution

by:
junfeb earned 10 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

Networking for the Cloud Era

Join Microsoft and Riverbed for a discussion and demonstration of enhancements to SteelConnect:
-One-click orchestration and cloud connectivity in Azure environments
-Tight integration of SD-WAN and WAN optimization capabilities
-Scalability and resiliency equal to a data center

Question has a verified solution.

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

Suggested Solutions

Title # Comments Views Activity
Fixing Non-Standard characters in text 8 111
Foxpro errors 4 42
corrupt Databases 9 65
Display SQL 2008 last modified/update Database 11 16
I guess that all of us know that caching the data usually increase the performance, but I worried if all of us are aware about the risk that caching the data provides and how to minimize this.  That’s the reason why I decided to write this short art…
When table data gets too large to manage or queries take too long to execute the solution is often to buy bigger hardware or assign more CPUs and memory resources to the machine to solve the problem. However, the best, cheapest and most effective so…
Video by: Steve
Using examples as well as descriptions, step through each of the common simple join types, explaining differences in syntax, differences in expected outputs and showing how the queries run along with the actual outputs based upon a simple set of dem…
Polish reports in Access so they look terrific. Take yourself to another level. Equations, Back Color, Alternate Back Color. Write easy VBA Code. Tighten space to use less pages. Launch report from a menu, considering criteria only when it is filled…

838 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