33 lines
1.1 KiB
Markdown
33 lines
1.1 KiB
Markdown
# lec20
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_more on mapping cardinalities_
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## Pariticipation Constraints
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These are _perscriptive_ constraints.
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* total
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If _all_ the entities in a set participate we say that the participation is total for that set.
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* partial
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If even one entity in the set is not participating then the pariticpation is _partial_ for that set.
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## Entity Keys
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If we have a table for a relationship, we can identify all the relationships if we can uniquely identify enties from either given set.
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Essentially if we can identify both(all) pariticipants in a given relationship table we can find any relationship in our relation-set.
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## Weak Entity Sets
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Any set where we can not uniquely all entities in that set.
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Let's say we have a tournament.
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We'll have players, with some _name_ and _jersey number_.
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We also have teams with a _team-name_ and likely some _team-id_.
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This means our players entity set is a weak set, but, because _all_ players participate in a team by definition of what a player is.
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Furthermore we may use the relationship between teams and players to idetify players.
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