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  1. #1
    Join Date
    Oct 2007
    Posts
    19

    Hello and Question about Entities

    Hello everyone

    I have joined the forum as I am interested in learning more about databases.

    In my work I look after various MySQL databases, which are connected to our web site.

    Recently I have decided to complete a degree in IT and one of the courses I am taking (while still working) is a database course.

    I am currently trying to absorb the theory of database design, which is really interesting, especially as I can see reasons behind certain designs and tables in our databases.

    I am finding some of it difficult though, for example differences between weak and associative entities and when to implement each. Any feedback on that would be very welcome

    I have Modern Database Management by Hoffer et al and another book by Connolly and Begg

  2. #2
    Join Date
    Apr 2002
    Location
    Toronto, Canada
    Posts
    20,002
    can you give an example of a weak entity, and an example of an associate entity

    they are for different purposes, i think, so i don't see how you would need to choose one or the other
    rudy.ca | @rudydotca
    Buy my SitePoint book: Simply SQL

  3. #3
    Join Date
    Oct 2007
    Posts
    19

    Weak and Associative Entities

    Hi thanks for replying. I have looked and can see the difference, you are right they are for different purposes.

    I have just tried to digest a huge weekly chunk of information, including mapping Entity Relationship Diagrams to tables.

    I got confused about the difference between a weak and an associative entity. So whereas a weak entity would only depend on another, strong entity for existence (ie an Employee Table with Dependent as the weak entity), an Associative Entity would change a many to many relationship by adding another table. So for example course and student, which is a many to many relationship could have another entity - enrolment, which would create one to many relationships.

    Sometimes it helps to just speak about something that seems confusing, sorry for the newbie question!

    I am enjoying learning the theory, but some of it is heavy going. I work from day to day with MySQL, but have never known any of the theory.

    Today I downloaded OracleXE as a trial.

  4. #4
    Join Date
    Apr 2002
    Location
    Toronto, Canada
    Posts
    20,002
    the way i understand a weak entity, its PK includes the FK to the parent, while a strong identity's PK doesn't

    and the associative entity is exactly as you said, implementing a many-to-many relationship
    rudy.ca | @rudydotca
    Buy my SitePoint book: Simply SQL

  5. #5
    Join Date
    Oct 2007
    Posts
    19

    Weak Entities

    Wasn't quite sure how the first bit works, but probably need to learn more about Primary and Foreign Keys!

    By the way, just read your article on relational integrity. Very interesting and helpful. I did not even know it could be implemented and have not used it in the MySQL tables that I am using, although I can see how it would be very helpful, we have tables of members, with related addresses, subscriptions and payments and sometimes these lose the relationships. Well often really!

    I just try to keep on top of it by doing queries to pick up duplicates and so on. Luckily there are only 3,000 or so members' records.

    Generally I really enjoy my work with databases and enjoy finding ways to query and extract data. Doing this course I am getting moments or "oh right, that is how it is done" or "oh right that is how it could be done"

  6. #6
    Join Date
    Oct 2012
    Posts
    1

    reply

    Week entity depends on the associative entity.Suppose you take admission and you are informed your class will be held in Room 5 .here if Room No 5 Exist then your class would be held.Here room5 is Associative entity and your class is weak entity.

    Regards
    helen
    Nitrotek

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