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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?
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.
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