An entity set that does not have a primary key is referred to as a weak
entity set.
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The existence of a weak entity set depends on the existence of a
strong entity set; it must relate to the strong entity set via a one-to-many
relationship set.
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A discriminator of a weak entity set is the set of attributes
that distinguishes it among all the entities of a weak entity set.
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The primary key of the weak entity set is formed by the primary key of
the strong entity set on which the weak entity set is existence
dependent, plus the weak entity set's discriminiator.
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The relationship set which links the weak identity set with the strong
identity set is referred to as the identifying entity set.
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All weak entities need to be identified by the identifying entity set.
We thus speak of a total participation of the weak entity set
in the relationship.
Consider the diagram to the right.
The entity set "Transaction" should be considered as a weak entity set,
because a "Transaction" can not exist without an "Account" entity.The
"Log" relationship set serves as an identifying entity set. The
attribute "transaction number" is the discriminator of the "Transaction"
entity set.
Express these facts in the diagram:
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Hold the mouse down for a second on the "Transaction" entity set to
open the popup menu. In the popup menu choose "Type > Weak".
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Open the popup menu for the relationship set "Log". Choose "Type >
Identifying".
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Hold the mouse down for a second on the connection between
"Account" and "Log" to open the popup menu. In the popup menu choose
"Cardinality > 1".
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Open the popup menu for the connection between "Account" and "Log".
Choose "Mark as total participation".
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Open the popup menu for the attribute "transaction-number". Choose
"Discriminating".
Click at "Compare with Solution" to see whether your diagram is
correct, and to get a description of the solution.
Source:
http://www.cs.sfu.ca/CC/354/zaiane/material/slides/Chapter2/sld024.htm