AFM241 Study Guide - Midterm Guide: Unique Key

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In any given table, there are a number of combinations or single attributes that could be chosen as
primary keys
However, from this list we only choose one as the primary key
The remaining keys that were not selected are called candidate keys and are not chosen
due to some impracticality
It’s easy to search for specific records in a table if one knows the primary key, but if you
don’t know the key what to do?
E.g. in BlueBikes employee table, if you don’t have the employee number (primary
key), you can search them up with their first or last or a combination of both. This is
called a secondary key, and it is not necessarily unique
5.3.4 Associations Between Entities
When we analyze the associations between occurrences of different entities it is customary to refer to
them as relations
A relation between any two entities is defined as a pair of associations that exist between the two of
them
There are three types of relations:
One to one (1:1)
One to many (1:M)
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Document Summary

In any given table, there are a number of combinations or single attributes that could be chosen as primary keys. However, from this list we only choose one as the primary key. The remaining keys that were not selected are called candidate keys and are not chosen due to some impracticality. E. g. in bluebikes employee table, if you don"t have the employee number (primary key), you can search them up with their first or last or a combination of both. This is called a secondary key, and it is not necessarily unique. When we analyze the associations between occurrences of different entities it is customary to refer to them as relations. A relation between any two entities is defined as a pair of associations that exist between the two of them. It is important to examine relations from both directions (a b and b a)

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