AFM241 Study Guide - Midterm Guide: Conceptual Schema, Unique Key

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E.g. a product from the product table can correspond to an entry in the order line table but
it doesn’t need to, as there are products that might never be sold. Therefore, optional
A mandatory relation occurs when occurrences on both sides of a relation must exist.
E.g. the relation between tOrder and tOrderline is mandatory because an invoice must have
line items for it to exist
A weak entity meets two criteria:
Its existence is dependent on another entity e.g. a line item cannot exist without a
corresponding invoice
Its primary key is at least partially derived from the related entity’s primary key e.g. the
primary key of the tOrderline is a combination primary key, and order number (primary key
of the order table) is part of the combination
If it doesn’t meet the above two criteria, then it’s a strong entity
Finally, a recursive relation occurs when an entity is related to itself
In this table we have the list of employees and the reporting structure. This will allow us to see who is
supervising who
5.3.5 entity relationship diagram
First step in the process of designing a database is to create a conceptual data model- a model that
describes the structure of the data that will be stored in the database
Focuses on how data will be represented, organized, and connected
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