What is a Schema? & Types of Schemas?

Graphical Representation of the datastructure.

First Phase in implementation of Universe        

There are three types of schema 

Out of three schema's two schemas will be used in real ime and one more is for knowledge purpose

1.Star Schema          -Real Time Usage

2.Snowflake Schema   -Real Time Usage

3.Fact Constellation Schema -Knowledge Purpose

  What is a star schema? 

   Star schema is a data warehouse schema where there is only one  "fact table" and many denormalized dimension tables.

Fact table contains primary keys from all the dimension tables and other numeric columns columns of additive, numeric facts.

         

 What is a snowflake schema? 

   Unlike Star-Schema, Snowflake  schema contain normalized dimension tables in a tree like structure with many nesting levels.

Snowflake schema is easier to maintain but queries require more joins.

What is a Fact Constellation/Galaxy schema? 

Fact constellation is also known as galaxy schema. It is nothing but a schema which contains multiple fact tables shares dimensions. It is a collection of star schemas which shares their dimension. So it is called as a galaxy schema.

Or

We can also say that the combination of both star and snowflake is nothing but galaxy schema or fact constellation

Interview FAQ's:

What does it mean by grain of the star schema? 

   In Data warehousing grain refers to the level of detail available in a given fact table as well as to the level of detail provided by a star schema.

It is usually given as the number of records per key within the table. In general, the grain of the fact table is the grain of the star schema.