1.1 KiB
lec20
more on mapping cardinalities
Pariticipation Constraints
These are perscriptive constraints.
- total
If all the entities in a set participate we say that the participation is total for that set.
- partial
If even one entity in the set is not participating then the pariticpation is partial for that set.
Entity Keys
If we have a table for a relationship, we can identify all the relationships if we can uniquely identify enties from either given set. Essentially if we can identify both(all) pariticipants in a given relationship table we can find any relationship in our relation-set.
Weak Entity Sets
Any set where we can not uniquely all entities in that set. Let's say we have a tournament.
We'll have players, with some name and jersey number. We also have teams with a team-name and likely some team-id.
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. Furthermore we may use the relationship between teams and players to idetify players.