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I've tried to look on Google about a decent explanation of weak and strong entity type, but I haven't fully understood them.
Could someone give me an example of a strong and weak entity type?
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Weak entities are represented with double rectangular box in the ER Diagram and the identifying relationships are represented with double diamond. Partial Key attributes are represented with dotted lines. Example-1: In the below ER Diagram, 'Payment' is the weak entity.
A strong entity is complete by itself and is not dependent on any other entity type. It possess a primary key which describes each instance in the strong entity set uniquely. That means any element in the strong entity set can be uniquely identified.
A strong or identifying relationship is when the primary key of the related entity contains the primary key of the “parent”. An entity is weak when two conditions are met: The entity is existence-dependent on another entity. The entity gets at least part of its primary key from that other entity.
The relationship of two strong entities is represented by a single diamond. Various strong entities, when combined together, create a strong entity set. A weak entity is dependent on a strong entity to ensure its existence. Unlike a strong entity, a weak entity does not have any primary key.
These are called weak entity types. In contrast Entity types that do have a key attribute are called strong entity types. Generally in any entity type we have lots of entities. To distinguish them we have key attributes to satisfy uniqueness constraint.
These are called Weak Entity type. The entity sets which do not have sufficient attributes to form a primary key are known as weak entity sets and the entity sets which have a primary key are known as strong entity sets. Attention reader! Don’t stop learning now.
Unlike a strong entity, a weak entity does not have any primary key. It instead has a partial discriminator key. A weak entity is represented by a double rectangle.
A weak entity is one that can only exist when owned by another one. For example: a ROOM can only exist in a BUILDING. On the other hand, a TIRE might be considered as a strong entity because it also can exist without being attached to a CAR.
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Just to play with it, question is strong entity type and answer is weak. Question is always there, but an answer requires a question to exist.
Example: Don't ask 'Why?' if Your Dad's a Chemistry Professor
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