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er diagrams

ER diagrmas introduction

QuestionAnswer
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