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Entities and Relationships
Tutoring > Databases > Entities and Relationships
Relationships need to exist between entities and between tables for a database to be useful, otherwise it would just be a collection of tables.
Cardinality of relationships
A relationship between one entity and another entity can be:
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Many to many - M:N e.g. Many cars can have many colours.
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One to many - 1:M e.g. One order can have many items on that order.
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One to one - 1:1 e.g. One person can have one unique ID number and only one ID number can be assigned to one person.
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This is the cardinality of the relationship.
Entities and entity types
Entities are the "things" in a table. Each entity should be a "thing" of an entity type.
Examples of entity types are:
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People.
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Employing organisations.
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Companies.
Strong and weak relationships
A relationship from entity type A to entity type B, mediated by having A’s primary key (PK) as a foreign key in B, is strong when B’s PK contains A’s PK.
A relationship is weak if it is not strong! Most relationships are weak.
A PK is a Primary Key. More on this in Keys.
Strong and weak entity types
An entity type E is weak when it is existence dependent on another entity type F. Therefore E -> F is MANDATORY.
Any entity type that gets only weak relationships is strong.
Usually entity types are strong.