table relationships rdbm. prof. leighton2 establishing table relationships rdbms allow us to...

Table Relationships

RDBM

Prof. Leighton 2

Establishing Table Relationships

• RDBMS allow us to establish relationships among tables

• Have a primary key in our REGIONS table and a foreign key in the STATE table

• Must link or join tables

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Three Types of Table Relationships

• One to one relationship

• One to many relationship

• Many to many relationship

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One to One Relationship

• Each record in one table has exactly one matching record in another table• Choose one table as the primary table with the primary key

• Split the REGIONS table into two tables• Table one: physical characteristics of the region such as the area

• Table two: information about home ownership rates

• Have the same number of records in each table• Tables share a one to one relationship

• But they contain different fields

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One to Many Relationship • REGIONS table contains a primary key,

Region ID• STATES table has foreign key,Region ID

• One record in the first table (the REGION table) matches many records in the second table (the STATES table)

• One record in the STATES table matches only one record in the REGIONS table

• If you join the two tables you will have as many records as you have in the table with the foreign key, the many side of the relationship

• Many may mean zero or only one. (For example, one employer could have advertised none or only one job.)

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One to Many Relationship, Cont..

• The one table, the table with the primary key, is the primary table

• The many table or the table with the foreign key is the related table

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Many to Many Relationship• One record in the first table matches many

records in the second table

• One record in the second table matches many records in the first

• These situations are handled by creating a number of one to many relationships• Example: registration at a university

• Many classes and many students

Establishing the Relationship

The Relationship

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Referential Integrity - Internal Consistency

• Can’t accidentally delete or change related data

• System of rules to ensure relationships between related tables are valid• Can’t enter a value in the foreign key field (related table) that doesn’t

exist in the primary key (primary table)

• Can’t delete record from a primary table if there are matching records in a related table

• Can’t change a primary key value in primary table if that record has related records

• Enforce referential integrity is an option

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Override Referential Integrity

• Set referential integrity on

• Override• Cascade deletes - when you delete a record from a primary table, Access

automatically deletes from all related tables those records with a foreign key value equal to the primary key value you deleted

• Cascade updates - when you change a primary key value, Access automatically changes all related tables’ foreign key values that equal the primary key value

Override Referential Integrity

Print the Relationship

Print the Relationship

The End