Unary Relationships

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Unary Relationships
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On the Job

• Is this a legitimate design?
• Do you see problems with it?

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The Family

• Is this a legitimate design?
• Do you see problems with it?

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Complicated RelationshipsAmong People

• Workers have supervisors, who also have
  supervisors
• Men and women can marry more than once
• Children born out of wedlock
• Half-brothers and -sisters
• What design will accommodate all of these?

Not intended as a comment on social mores.
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Relating an Entity to Itself

• An entity can be related to itself.
• Called a unary or recursive relationship
• Where do you put the foreign keys?

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Unary 1M

• Recursive foreign key placed in entity
• Name of FK must be different from name of PK
• Second instance of entity required in queries and
  relationships screen

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Unary 11 or MM

• Associative entity required

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Unary 11 or MM

• Name of FKs must be different in associative
  entity
• Second instance of entity required in queries and
  relationships screen

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Common Uses of Unary Relationships

• People
• sometimes supertype/subtype required to fully
  document relationship
• Bill of Materials
• products composed of other products
• Political Geography
• Precincts within cities within counties within
  states within countries within continents