An entity set that does not have a primary key is referred to as a weak entity set.

• 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.
• A discriminator of a weak entity set is the set of attributes that distinguishes it among all the entities of a weak entity set.
• 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.
• The relationship set which links the weak identity set with the strong identity set is referred to as the identifying entity set.
• 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.

 

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Source: http://www.cs.sfu.ca/CC/354/zaiane/material/slides/Chapter2/sld024.htm