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er diagrams
ER diagrmas introduction
| Question | Answer |
|---|---|
| Entity | Represents a place, a thing, a person you want to track in a database. This will become a table in a database |
| entity instance | Each occurrence of the entity |
| attribute | Describes various characteristics about an individual entity |
| primary key | A special attribute that make it so that no two rows will have the same value for that attribute |
| relationship | Describes how one or more entities interact with each other |
| cardinality | The count of instances that are allowed or are necessary between entity relationships |
| ERD Entity Relationship diagram | Graphical representation of the data requirements for a data base |
| composite key | When you need more than one attribute to make each instance unique |
| mandatory | means that you must have at least one instance in the relationship |
| mandatory | means that you must have at least one instance in the relationship |
| Entity | basically is a container: all the elements that are like it are grouped together under it |
| Generalization | in a “bottom up” design, is an entity that represents a superset of other entities |
| Specialization | in a “top down” design, is an entity that represents a subset of another entity |
| incomplete | Specialization where only some individuals of the parent entity are specialized |
| complete | Specialization where all individuals of the parent entity are specialized (have one or more unique attributes that are not common to the parent). |
| overlapping | Specialization where an individual of the parent entity may be a member of more than one of the specialized entities. |
| disjoint | (also called an exclusive specialization). When an individual of the parent entity may be a member of only one specialized entities. |
| one to many relationship | In a one-to-many relationship between Table A and Table B, each row in Table A is linked to 0, 1 or many rows in Table B |
| many to many relationship | Refers to the relationship between two entities A and B in which A may contain a record for which there are many records B and vice versa |
| one to one relationship | relationship between two entities A and B in which one element of A may only be linked to one element of B, and vice versa |
Created by:
annap
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