Example Ales are a kind of beer Not every beer is an ale but some are Suppose

Example ales are a kind of beer not every beer is an

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Example. Ales are a kind of beer. Not every beer is an ale, but some are. Suppose that in addition to all the properties (attributes and relationships) of beers, ales also have the attribute color .
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13 Subclasses Subclass = special case = fewer entities = more properties. Assume subclasses form a tree, i.e., no multiple inheritance. Isa triangles indicate the subclass relationship, which points to the superclass. Example. Ales are a kind of beer. Not every beer is an ale, but some are. Suppose that in addition to all the properties (attributes and relationships) of beers, ales also have the attribute color . Beers Ales isa name manf color
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14 Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key.
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15 Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes.
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16 Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set.
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17 Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s).
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18 Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s). Courses dept number hours room
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19 Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s). Courses dept number hours room Note that hours and room could also serve as a key, but we must select only one key .
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20 Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s). In an Isa hierarchy, only the root entity set has a key, which is the key for all entities in the hierarchy.
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