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Unary Relationships

Unary Relationships. On the Job. Is this a legitimate design? Do you see problems with it?. The Family. Is this a legitimate design? Do you see problems with it?. Complicated Relationships Among People*. Workers have supervisors, who also have supervisors

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Unary Relationships

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  1. Unary Relationships

  2. On the Job • Is this a legitimate design? • Do you see problems with it?

  3. The Family • Is this a legitimate design? • Do you see problems with it?

  4. 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.

  5. 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?

  6. Unary 1:M • 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

  7. Unary 1:1 or M:M • Associative entity required

  8. Unary 1:1 or M:M • Name of FKs must be different in associative entity • Second instance of entity required in queries and relationships screen

  9. 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

  10. More about the 1:1 or M:M Interpersonal Unary Relationship • Interpersonal relationships are often two-way, with the reciprocal relationship sometimes having a different name • Parent-child • Sibling-sibling • Spouse-spouse • Supervisor-worker • Employer-employee

  11. More about the 1:1 or M:M Interpersonal Unary Relationship • In general, it is better to have only one record for a given relationship • Do not have • One record where person A is “child of” B • A second record where B is “parent of” A • Instead • A single record that records both directions of the relationship • Requires that there be the ability to indicate the reciprocal of a relationship

  12. A Solution for the 1:1 or M:M Interpersonal Unary Relationship A “Relationship Type” table is created to hold both “forward” and “reciprocal” terms for the relationship

  13. A Solution for the 1:1 or M:M Interpersonal Unary Relationship • Only one entry per pair (if 1:1)

  14. A Solution for the 1:1 or M:M Interpersonal Unary Relationship • Carefully define the directionality • In these examples • [second person] is the [relationship] of [first person] • [first person] is the [reciprocal] of [second person]

  15. A Solution for the 1:1 or M:M Interpersonal Unary Relationship • Create a view for one direction that includes PersonID, Name, and Relationship with Other Person

  16. A Solution for the 1:1 or M:M Interpersonal Unary Relationship • Expression that returns the “Relationship with Other Person” Relationship: [tblRelationshipType]![RelationshipType] & " of " & fnNameFirstLast([tblPerson_1]![FirstName], [tblPerson_1]![MidName],[tblPerson_1]![LastName])

  17. A Solution for the 1:1 or M:M Interpersonal Unary Relationship • Create a view for the reciprocal direction

  18. A Solution for the 1:1 or M:M Interpersonal Unary Relationship Data entry form allows for the specification of relationship in one direction

  19. A Solution for the 1:1 or M:M Interpersonal Unary Relationship Already-entered reciprocal relationships are shown to discourage duplicate entries

  20. A Solution for the 1:1 or M:M Interpersonal Unary Relationship • A UNION query returns all records from both views SELECT * FROM vueSecondPersonRelationship UNION SELECT * FROM vueFirstPersonRelationship; • Must be entered in SQL view • Normally, UNION query eliminates duplicate rows

  21. A Solution for the 1:1 or M:M Interpersonal Unary Relationship • A UNION query returns all records from both views There is only one relationship record for persons 1 and 2, but both directions of the relationship are returned

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