Weak Entity Set Notation

Weak Entity Set Notation

We shall adopt the following principles to show that an entity set is weak and to declare its key attributes.

1. If an entity set is weak, it will be shown as a rectangle with a double border. Examples of this principle are Crews in figure "A weak entity set for crews, and its connections" and Contracts in figure "Connecting entity sets are weak" of "Weak Entity Sets".

2. Its supporting many-one relationships will be shown as diamonds with a double border. Examples of this principle are Unit-of in figure "A weak entity set for crews, and its connections" and all three relationships in figure "Connecting entity sets are weak" of "Weak Entity Sets".

3. If an entity set provides any attributes for its own key, then those attributes will be underlined. An example is in figure "A weak entity set for crews, and its connections" of "Weak Entity Sets", where the number of a crew participates in its own key, though it is not the complete key for Crews.

We can summarize these principles with the following rule:

Whenever we use an entity set E with a double border, it is weak. E's attributes that are underlined, if any, plus the key attributes of those entity sets to which E is attached by many-one relationships with a double border, must be unique for the entities of E.

We should keep in mind that the double-diamond is used only for supporting relationships. It is possible for there to be many-one relationships from a weak entity set that are not supporting relationships, and therefore do not get a double diamond.

Example : In figure "Connecting entity sets are weak" of "Weak Entity Sets", the relationship Studio-of need not be a supporting relationship for Contracts. The reason is that each movie has a unique owning studio, determined by the (not shown) many-one relationship from Movies to Studios. Therefore, if we are told the name of a star and a movie, there is at most one contract with any studio for the work of that star in that movie. In terms of our notation, it would be correct to use an ordinary single diamond, rather than the double diamond, for Studio-of in figure "Connecting entity sets are weak" of "Weak Entity Sets".



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