Data Warehouse - Schemas

Data Warehouse – Schemas

A schema is outlined as a logical description of database the place truth and dimension tables are joined in a logical method. Information Warehouse is maintained within the type of Star, Snow flakes, and Truth Constellation schema.

Star Schema

A Star schema comprises a truth desk and a number of dimension tables. Every dimension is represented with solely one-dimension desk and they don’t seem to be normalized. The Dimension desk comprises a set of attributes.


  • In a Star schema, there is just one truth desk and a number of dimension tables.
  • In a Star schema, every dimension is represented by one-dimension desk.
  • Dimension tables are usually not normalized in a Star schema.
  • Every Dimension desk is joined to a key in a truth desk.

The next illustration reveals the gross sales knowledge of an organization with respect to the 4 dimensions, particularly Time, Merchandise, Department, and Location.

Star Schema

There’s a truth desk on the heart. It comprises the keys to every of 4 dimensions. The very fact desk additionally comprises the attributes, particularly {dollars} bought and items bought.

Notice − Every dimension has solely one-dimension desk and every desk holds a set of attributes. For instance, the situation dimension desk comprises the attribute set {location_key, avenue, metropolis, province_or_state, nation}. This constraint might trigger knowledge redundancy.

For instance − “Vancouver” and “Victoria” each the cities are within the Canadian province of British Columbia. The entries for such cities might trigger knowledge redundancy alongside the attributes province_or_state and nation.

Snowflakes Schema

Some dimension tables within the Snowflake schema are normalized. The normalization splits up the info into extra tables as proven within the following illustration.

Snowflakes Schema

Not like within the Star schema, the dimension’s desk in a snowflake schema are normalized.

For instance − The merchandise dimension desk in a star schema is normalized and cut up into two dimension tables, particularly merchandise and provider desk. Now the merchandise dimension desk comprises the attributes item_key, item_name, kind, model, and supplier-key.

The provider secret is linked to the provider dimension desk. The provider dimension desk comprises the attributes supplier_key and supplier_type.

Notice − As a result of normalization within the Snowflake schema, the redundancy is lowered and due to this fact, it turns into simple to take care of and the save space for storing.

Truth Constellation Schema (Galaxy Schema)

A truth constellation has a number of truth tables. It is usually often known as a Galaxy Schema.

The next illustration reveals two truth tables, particularly Gross sales and Delivery −

Galaxy Schema

The gross sales truth desk is similar as that within the Star Schema. The transport truth desk has 5 dimensions, particularly item_key, time_key, shipper_key, from_location, to_location. The transport truth desk additionally comprises two measures, particularly {dollars} bought and items bought. It is usually doable to share dimension tables between truth tables.

For instance − Time, merchandise, and placement dimension tables are shared between the gross sales and transport truth desk.