Data Modeling - Entity (Object, Table)

1 - About

Entity is:

  • A concept to abstractly represent all instances of a group of similar “things”
  • Class of persons, places, objects, events, or concepts about which we need to capture and store data.
  • A person, place, event, or thing about which data is collected data is collected

Entity must be multiple occurrences (of instance) to be an entity An entity instance is a single occurrence of an entity.

An entity is commonly implemented by a table and an instance by a row.

In relational maths, the terms:

  • Table means a relation.
  • Row means a tuple.
  • Column means an attribute.

3 - Categories/classification of entities

At a high level:

  • transaction entities correspond to fact tables
  • component entities correspond to dimension tables
  • classification tables correspond to hierarchy tables

3.1 - Transaction/Facts Entities

These entities record details of business events (e.g., orders, shipments, payments, insurance claims, bank transactions, hotel bookings, airline reservations, and hospital admissions).

Generally, this tables are at the many end of a all one-to-many relationship.

3.1.1 - Fact Dimension / Master detail transaction table

One exception to this rule is a fact dimension which has a one-to-many relationship (master detail) with the facts entities but still describe business events.

For instance, an Order Header and an Order Item table describe the same business event but with different levels of detail. The Order item table has the more detailed data whereas the Order header is grouped by order number and therefore its attributes can be in a dimensional model degenerated

This transaction entities are connected in a master-detail structure, they should be combined into a single fact table, as they represent different views of the same event. The split into “master” and “detail” is simply a requirement of normalization (1NF).

All attributes of the master record should be allocated down to the item level if possible (Kimball, 1995). For example, if discount is defined at the master (Order) level, the total discount amount should be allocated at the item level (e.g., in proportion to the price for each item). The same should be done to delivery charges, order level taxes, fees, etc.

3.2 - Snapshot Entities

This entities recording a static level of some commodity at a point in time (e.g., account balances, customer sold and inventory levels). These do not record business events as such, but the effect of events on the state of an entity.

For instance, a stock level table which records a static level.

3.3 - Component Entities

These entities are directly related to a transaction entity by a one-to-many relationship.

3.4 - Classification / Lookup Entities

These entities are related to a component entity by a chain of one-to-many relationships. These define embedded hierarchies in the data model and are used to classify component entities.

3.5 - Others

Some of the entities do not fit into any of these categories. Such entities do not fit the hierarchical structure of a dimensional model and therefore cannot be represented in the form of a star schema and must be “dimensionalized”.

3.6 - Dates and Times Dimension

Date and/or Time appear as explicit dimensions in most star schemas to support different types of historical analyses. These are not normally represented as entities in operational systems.

Date and Time are usually represented as separate dimensions to reduce the size of the dimension tables (Kimball, 1996).

Dates must be explicitly modeled in a dimensional schema, whereas at the operational level they are represented as data types.

4 - Other Notations

4.1 - Mathematical Notation: A set

The mathematical concept of a set is just a group of unique items, meaning that the group contains no duplicates.

4.2 - Programming language: A collection

5 - Naming Convention

  • The name of a database table is often plural (for example, owners, pets, specialities, and so on).
  • The name of the column contains a prefix of its table name.
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data_modeling/entity.txt · Last modified: 2017/02/12 21:06 by gerardnico