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Weak and Strong Entities

Posted on 1998-11-27
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Description of Weak and Strong Entities.
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Question by:majorjohn
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by:pjmaze
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hi,

The existance of a weak entity depends on the existence of a strong entity; it must relate to the strong via a on-to-many relationship set.

The primary key of a weak entity is formed by the primary key of the strong entity on which the weak entity set is existence depend, plus the weak entity set’s discriminator (partial Key).



A strong entity is one that has its own primary key and dont depand on any other entitie.

If you need more info, ask ..

see ya
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by:majorjohn
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Yes I need some more information with example.

Thanks,
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by:camazorro
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Hi MajorJohn

Typical Expample of weak and strong entity

EMPLOYEE and DEPENDANTS

The entity DEPENDANTS depends of the entity EMPLOYEE
DEPENDANTS = Wife, childrens, family.

In a Company DB the DEPENDANTS entity can exist by it self. In these case we make the relation DEPENDANTS --<depends>-- EMPLOYEE

Bye Bye
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by:P_S_Price
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The above model may not be a true example. It could also be possible for a dependent to be an Employee!! Nepotism is best kept in the family!!
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traygreen earned 10 total points
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Strong entity...
Table Orders
ID        int      Primary Key
OrderNum  varchar
OrderDate datetime

Table Items
ID        int      Primary Key
Desc      varchar

weak....
Table OrderDetail
OrderID   int      Foreign Key (references Order.ID)
ItemID    int      Foreign Key (references Item.ID)
Qty       int

In this example the items and orders tables are core level or "Stong" entities.  They stand alone without requiring other defined objects.
The Order detail is a "Weak" entity as it requires the existence of both an order and item to exist.

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