What is a Dimension Table and Types of Dimension table

What is a Dimension Table:

If a Table contains “primary keys” and it gives the detailed information about business then such a table is called dimension table.

A Dimension table is a table which holds a list of attributes or qualities of the dimension most often used in queries and reports.

E.g. “Store” dimension can have attributes

Street,

Block Number,

City,

Region,

Country where it is located in addition to its name.

Dimension tables are ENTRY POINTS into the fact table.

1. The number of rows selected and processed from the fact table depends on the conditions (“WHERE” clauses) the user applies on the dimensional attributes selected.

2. Dimension tables are typically DE-NORMALIZED in order to reduce the number of joins in resulting queries. Dimension table attributes are generally STATIC, DESCRIPTIVE fields describing aspects of the dimension Dimension tables typically designed to hold IN-FREQUENT CHANGES to attribute values over time using SCD concepts Dimension tables are TYPICALLY used in GROUP BY SQL queries Every column in the dimension table is TYPICALLY either the primary key or a dimensional attribute Every non-key column in the dimension table is typically used in the GROUP BY clause of a SQL Query.

Types of Dimension table:

1.Conformed Dimension: 
If a dimension table is shared by multiple fact tables then that dimension is known as conformed dimension table.

2.Junk dimension: 
Junk dimensions are dimensions that contain miscellaneous data like flags, gender, text values etc and which are not useful to generate reports.

3.Slowly changing dimension: 
If the data values are changed slowly in a column or in a row over the period of time then that dimension table is called as slowly changing dimension. 

Ex: Interest rate, Address of customer etc 

There are three types of SCD’s:

Type – 1 SCD: A type-1 dimension keeps the most recent data in the target.

Type – II SCD: keeps full history in the target. For every update it keeps a new record in the target.

Type – III SCD: keeps the current and previous information in the target (partial history).

Note: To implement SCD2 we use Surrogate key.

Example1:

Example2:

        Surrogate key:

1. Surrogate Key is an artificial identifier for an entity. In surrogate key values are generated by the system sequentially (Like Identity property in SQL Server and Sequence in Oracle). They do not describe anything.

2. Joins between fact and dimension tables should be based on surrogate keys

3. Surrogate keys should not be composed of natural keys glued together

4. Users should not obtain any information by looking at these keys

5. These keys should be simple integers

6. Using surrogate key will be faster

7. Can handle Slowly Changing dimensions

4.Degenerated dimension: 
A degenerate dimension is data that is dimensional in nature but stored in a fact table. For example, if you have a dimension that only has Order Number and Order Line Number, you would have a 1:1 relationship with the Fact table. Therefore, this would be a degenerate dimension and Order Number and Order Line Number would be stored in the Fact table. Fast Changing Dimension: A fast changing dimension is a dimension whose attribute or attributes for a record (row) change rapidly over time. Example: Age of associates, Income, Daily balance etc.

5.Roll playing dimension:

One dimension plays multiple roles to retrieve data from fact tables.

6.Dirty dimension:

In this dimension table records are maintained more than once by the difference of non-key attributes.