An entity type that does not have any key attribute of its own is called a weak entity type. On the other hand, an entity type that has a key attribute is called a strong entity type. The weak entity is also called a dependent entity as it depends on another entity for its identification. The strong entity is called an independent entity, as it does not rely on another entity for its identification. The weak entity has no meaning and relevance in an E-R diagram if shown without the entity on which it depends. For example, consider an entity type EDITION with attributes Edition_no and Type (Indian or International). It depends on another entity type BOOK for its existence, as an edition cannot exist without a book. Thus, EDITION is a weak entity type and BOOK is a strong entity type.
A weak entity does not have its own identifying attribute that can uniquely identify each instance. It has a partial identifying attribute, which is known as the partial key (or discriminator). A partial key is a set of attributes that can uniquely identify the instances of a weak entity type, which are related to the same instance of a strong entity type. For example, the attribute Edition_no of the entity type EDITION can be taken as a partial key.
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