Dimensional Modelling in DWH



Data Warehouse Dimensional Modelling (Types of Schemas)
Four types of schemas

Star Schema:

A star schema is the one in which a central fact table is sourrounded by denormalized dimensional tables.
A star schema can be simple or complex.
A simple star schema consists of one fact table where as a complex star schema have more than one fact table.

Snow Flake Schema:

A snow flake schema is an enhancement of star schema by adding additional dimensions.
Snow flake schema are useful when there are low cardinality attributes in the dimensions.


Galaxy Schema:
Galaxy schema contains many fact tables with some common dimensions (conformed dimensions). 
This schema is a combination of many data marts.

Fact Constellation Schema:

The dimensions in this schema are segregated into independent dimensions based on the levels of hierarchy.

For example, if geography has five levels of hierarchy like teritary, region, country, state and city;
constellation schema would have five dimensions instead of one.

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