What is a Dimension?

A measure or dimension is normally an additive numerical value that represents a business metric.  You are also not limited to a single measure.  You can have multiple measures within a fact table.  For example, if your fact table is used to track purchases internationally you might have measures for each type of currency.  If you are building a fact table for the retail industry you might also have the following measures cost, list price, average sale price.

Dimensions describe the objects involved in a business intelligence effort. While facts correspond to events, dimensions correspond to people, items, or other objects. For example, in the retail scenario, we discussed that purchases, returns and calls are facts. On the other hand, customers, employees, items and stores are dimensions and should be contained in dimension tables.

 

Dimension tables contain details about each instance of an object. For example, the items dimension table would contain a record for each item sold in the store. It might include information such as the cost of the item, the supplier, color, sizes, and similar data.

Dimension tables are typically small, ranging from a few to several thousand rows. Occasionally dimensions can grow fairly large, however. For example, a large credit card company could have a customer dimension with millions of rows. Dimension table structure is typically very lean, for example customer dimension could look like following:

Customer_key
Customer_full_name
Customer_city
Customer_state
Customer_country

Fact tables and dimension tables are related to each other. Again returning to our retail model, the fact table for a customer transaction would likely contain a foreign key reference to the item dimension table, where the entry corresponds to a primary key in that table for a record describing the item purchased.

Most data warehouses will have one or multiple time dimensions. Since the warehouse will be used for finding and examining trends, data analysts will need to know when each fact has occurred. The most common time dimension is calendar time. However, your business might also need a fiscal time dimension in case your fiscal year does not start on January 1st as the calendar year.

Most data warehouses will also contain product or service dimensions since each business typically operates by offering either products or services to others. Geographically dispersed businesses are likely to have a location dimension.

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