Courses dept number hours room Note that hours and room could also serve as a

Courses dept number hours room note that hours and

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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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21 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. Courses dept number hours room Beers Ales isa name manf color Note that hours and room could also serve as a key, but we must select only one key .
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22 Weak Entity Sets Occasionally, entities of an entity set need “help” to identify them uniquely.
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23 Weak Entity Sets Occasionally, entities of an entity set need “help” to identify them uniquely. Entity set E is said to be weak if in order to identify entities of E uniquely, we need to follow one or more many- one relationships from E and include the key of the related entities from the connected entity sets.
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24 Example (player) name is almost a key for football players, but there might be two players with the same name.
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25 Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number.
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26 Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number. But number , together with the (team) name related to the player by relationship Plays-on should be unique.
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27 Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number. But number , together with the (team) name related to the player by relationship Plays-on should be unique. Players Teams Plays-on name name number
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28 Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number. But number , together with the (team) name related to the player by relationship Plays-on should be unique. Players Teams Plays-on name name number Double diamond for supporting many-one relationship. Double rectangle for the weak entity set .
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29 Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets.
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30 Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets.
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