Foundation: The Date Dimension
Similar to the Product and Customer dimensions, the Date dimension is meant to provide context to the measures in the transaction table. Date dimensions typically have many attributes describing each date (e.g. Weekday, Week number, month name, quarter etc.). Say you wanted the sum of all sales that occurred in the second quarter of 2015. You would first need to find out the start and end dates of that quarter and then query the data based on that. However, with a date dimension, we can simply group by Q2 2015 as an attribute value and obtain our results. Nearly every fact table will have a date dimensions related to it.