Getting Started with Date Dimensions
The date dimension is the table that controls your time analysis and is the most important to get right, especially with PowerBI, Excel, and other visualization tools. Having various tags and descriptors that aid in your analysis gives your end users several options in creating their visualizations in addition to a base for DAX time intelligence calculations.
Note: The following article uses Excel as the starting point and provides the example available on Github
Why is a Good Date Dimension Table Important?
A good date dimension table is an essential component of any analytic solution. It provides a standardized and consistent way to store and reference important date-related information, such as dates, days of the week, months, quarters, and years.
With a well-designed date dimension table, users can easily perform time-based analysis, track trends, and make accurate forecasts. The table can also be used to calculate metrics such as year-over-year growth, quarter-over-quarter changes, and other key performance indicators.
Additionally, a good date dimension table can help ensure data integrity and accuracy by enforcing data type standards, providing consistency in date formats, and accommodating time zones and daylight saving time changes.
Overall, having a robust and comprehensive date dimension table is crucial for any organization that wants to gain valuable insights from their data and make data-driven decisions.
One (Date) Key to Rule Them All
The first significant step is how to link the date table to other tables in your model. Using an integer surrogate key based on the date is the best way to do this. This format for the join key will avoid errors due to different date formats. Some columns have a date-time value so that they may look like the same date. However, they will not join together because the time is different.
The Excel calculation below uses the DateValue column and converts the date into an integer value. If you have a different date granularity, such as time to seconds, you can change the calculation to include those values. Even if you have a date table only down to the month level, creating a date column with each month’s first date will allow for easier DAX calculations.
CalendarID = (YEAR([DateValue]) *10000) + (MONTH([DateValue]) *100) + (DAY([DateValue])) DateValue= Calendar Date
Get the Data Types Right!!
When using Excel, most columns classify themselves as “General.” As you add in the formulas, change the column formats to the correct data types, text for text, date for date, and number for number columns. These will get picked up when you import the table into PowerBI.
Prepare the Slicers
In PowerBI, as with other tools, you will want totals or the ability to filter based on different parts of the date. By having a slicer for Year, Month, and Week Numbers, you can add flexibility to your visualizations and filters. You will also need numeric values to provide sort values, as many visualization tools will sort them alphabetically.
You will also need numeric values for some columns that are text formatted in order to provide sort values as they will sort them alphabetically. MonthNum and WeekDayNum are examples pictured above. PowerBI allows you to sort one column based on the value of another.
The Excel example is available on Github. This gets you the basic time dimension that will allow you to enhance your data model in PowerBI, as you can add in various time intelligence calculations using DAX.
In conclusion, a good date dimension table is a vital component in any business intelligence system. It provides a consistent and structured way to store time-based information, enabling users to perform time-based analysis and make accurate predictions easily.
With a well-designed date dimension table, you can track trends, monitor performance, and identify areas for improvement. Additionally, a good date dimension table can help ensure data accuracy and consistency, making it an invaluable resource for decision-making.
The key point is that investing time and effort in creating a high-quality date dimension table can pay significant dividends regarding the value it adds to an organization’s data analytics capabilities.
NOTE: If you want to use Time Intelligence in Power BI models, you must join the tables using data values. This link will take you to an MSLearning module that will explain more about Time Intelligence using Power BI and Data Dimension tables.