Relationships

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Relationships- Any significant association between two entities

Relationships are always two-way

Relationships have three characteristics/attributes

Optionality - is or might be.

Name

Cardinality - one or more ,one and only one.

A makes B: Each A may make one or more B; Each B must be made by one and only one A

May - dashed line

Must - solid line

Kinds of Relationships

A (many solid) to B (single dashed) - each A must own one and only one B | Each B may own one or more A's

Each Employee must have one and only one employer | Each Employer may have one or more employees

Each section must be of one and only one course | Each Course may have one of more sections

A (many dashed) to B (single solid) - Each A many have one and only one B | Each B must have one or more A's

Each student many have one and only one study group | Each study group must have one or more students

A (many solid) to B (single solid) - Each A must have one and only one B | Each B many have one or more A's

Each registration must be in one an only one schedule | Each schedule must hold one or more registrations

A (many dashed) to B (single dashed) - Each A may have one and only one B | Each B may have one or more A

Each student may have one and only one honorary | Each honorary may have one or more Students

A (single solid) to B (single dashed) - Each A must have one and only one B| Each B may belong to one and only one

Each directory entity must have one and only one Teacher | Each teacher may have one and only one directory entity

A (single solid) to B (single solid) - Each A must have one and only one B | Each B must have one and only one A

Every learner must be a student | Every student is a learner

A (single dashed) to B (single dashed) - Each A may have one and only one B | Each B may have one and only one A

Each teacher may have one and only one library carrel | Each library carrel may have one and only one teacher

Each employee may have one and only one position | Each position may have one and only one employee

A (many dashed) to B (many dashed) - Each A may have one or more B | Each B may have one or more A

Each student take one or more courses | Each course may be taken by one or more students

Each person may read one or more books | each book may be read by one or more persons

A (many solid) to B (many solid) - Each A must have one or more B's | Each B must have one or more A's

This is illegal, it does not exist.

Circular Relation