Data Flow

How the data flows from dimensions to fact table?

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Fact table and dimension tables communicate using a referential key. When ever you want to account one dimension, the respective fact table key will take you to that particular dimension and vice versa

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Genarally data flows goes to Dimension to Fact.
Because dimension tablecontains  Primary Key. Fact table contains foriegn Key.
Data will load first Dimension followed by the fact table.

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In a Data Warehouse Consider there are two tables, D and F, where F is a fact table and D is a dimension table.
A dimension table is the one which contains the Primary key and a Fact table is the one which contains corresponding Foreign Key.

So it is obvious that data will be firstly loaded in a Primary key table and then it will be loaded in the foreign key table, this means data will flow from Dimension table to a Fact table.

Another Explanation could be if you see the definition of a fact table, the source of the data is always a dimension table, if a fact table is deriving data from say three dimension tables, then the three dimension tables will be loaded and then as per the definition the corresponding foreign keys data will be loaded in the fact table.


  • Feb 24th, 2009

Data in reality does not flow from dimensions to facts or visa versa. Dimensions contain textual descriptive information in a business unit of an Enterprise. This kind of data is usually contained in the where clauses of your query. Informations such a Customer name,address etc is dimensional. Facts are additive, quantitative data or facts that relates to the dimensions. These are stored in  fact table (and have relationship with the dimensions). For example a customer may have specific sales data, in the form of qantities of certain items purchase at certain store locations at certian dates. In this typical scenario you would end up with a sales fact table in relationship with a few dimensions such as customer informaton,store location, date,product etc. As you can see these dimensions are the filters of your query when you are seeking to get at the facts. It is true that the Primary keys of the dimensions migrate to the fact table to become a cooporate primary key in the fact table. So in the ETL process dimensions are loaded first and then the PKs of the dimensions migrate to the Fact table to establish relationship between the dimensional attibuates  and the facts. In that simplistic way one could speak of a flow from dimensions to facts but in reality there is no flow only a relationship that facilitates the star schema and ultimately rapid query access to the facts through your particular BI presentation interface.

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