When you are connected to a flat file, relational, or extracted data source, Tableau provides a robust built-in date hierarchy for any date field.

To see this in action, continue with the Chapter 3 workbook, navigate to the Built-in Date Hierarchy sheet, and create a view similar to the one that was shown by dragging and dropping Number of Patient Visits to Rows and Date of Admit to Columns. The YEAR(Date of Admit) field on Columns will have a plus sign indicator, like this:

When you click it, the hierarchy expands by adding QUARTER(Date of Admit) to the right of the YEAR(Date of Admit) on Columns, and the view is expanded to the new level of the hierarchy:

The YEAR(Date of Admit) field now has a minus sign indicator that allows you to collapse the hierarchy back to the year level. The QUARTER field also has a plus sign, indicating that you can expand the hierarchy further. Starting with Year, the hierarchy flows as follows: Year | Quarter | Month | Day. When the field is a date and time, you can further drill down into Hour | Minute | Second. Any of the parts of the hierarchy can be moved within the view or removed from the view completely.

The hierarchy is made up of Date Parts, which is one of the three ways a date field can be used. When you right-click the date field in the view or by using the drop-down menu, you'll see multiple date options, as follows:

The three major date types are evident, though not explicitly labeled, in the menu:

• Date part: This field will represent a specific part of the date, such as the quarter or month. The part of the date is used by itself and without reference to any other part of the date. This means that a date of November 8, 1980, when used as a month date part, is simply November. The November that's selected in the view here represents all of the Novembers in the dataset, while the number of patient visits is the total for both 2017 and 2018:

• Date value: This field will represent a date value, but rolled up or truncated to the level you select. For example, if you select a date value of month, then November 8, 2018 gets truncated to the month and year, and is November 2018. You'll notice that November 2017 and November 2018 each have a separate value in the header and a distinct bar:

• Exact date: This field represents the exact date value (including time, if applicable) in the data. This means that November 8, 1980, 2:01 am is treated as distinct from November 8, 1980, 3:08 pm.

It is important to note that nearly any of these options can be used as discrete or continuous fields. Date parts are discrete by default. Date values and exact dates are continuous by default. However, you can switch between discrete and continuous as required to allow for flexibility in the visualization.

For example, you must have an axis (requiring a continuous field) to create a reference line. Also, Tableau will only connect lines at the lowest level of row or column headers. Using a continuous date value instead of multiple discrete date parts will allow you to connect lines across multiple years, quarters, and months.