Data blending is a powerful and innovative feature in Tableau. It allows you to use data from multiple data sources in the same view. Often these sources may be different types. For example, you can blend data from Oracle with data from Excel. You can blend Google Analytics data with a spatial file. Data blending also allows you to compare data at different levels of detail. Some advanced uses of data blending will be covered in Chapter 8, Digging Deeper: Trends, Clustering, Distributions and Forecasting. For now, let's consider the basics and a simple example.

Data blending is done at an aggregate level and involves different queries sent to each data source, unlike joining, which is done at a row level and involves a single query to a single data source. A simple data blending process involves several steps, as shown in the following diagram:

We can see the following from the preceding diagram:

• Tableau issues a query to the primary data source.
• The underlying data engine returns aggregate results.
• Tableau issues another query to the secondary data source. This query is filtered based on the set of values returned from the primary data source for dimensions that link the two data sources.
• The underlying data engine returns aggregate results from the secondary data source.
• The aggregated results from the primary data source and the aggregated results from the secondary data source are blended together in the cache.

It is important to note how data blending is different from joining. Joins are accomplished in a single query and results are matched row-by-row. Data blending occurs by issuing two separate queries and then blending together the aggregate results.

There can only be one primary source, but there can be as many secondary sources as you desire. Steps three and four will be repeated for each secondary source. When all aggregate results have been returned, Tableau will match the aggregate rows based on linking fields.

Linking fields are dimensions which are used to match data blended between primary and secondary data sources. Linking fields define the level of detail for the secondary source. Linking fields are automatically assigned if fields match by name and type between data sources. Otherwise, you can manually assign relationships between fields by selecting, from the menu, Data | Edit Relationships, as follows:

The Relationships window will display the relationships recognized between different data sources. You can switch from Automatic to Custom to define your own linking fields.

Linking fields can be activated or deactivated for blending in a view. Linking fields used in the view will usually be active by default, while other fields will not. You can, however, change whether a linking field is active or not by clicking the link icon next to a linking field in the Data pane.