Any part of a data analysis or data visualization requires the data itself to be connected. In Tableau, there are several ways of joining data if there are multiple files or multiple excel sheets to obtain an appropriate data model.
Tableau connects to data in the Data Source Page where you can select your first file to connect.
In this step, I have connected to an excel file called Bookshop that contains multiple sheets such as Author, Award, Book…
After dragging in the Book table to the Canvas, we have a live connection to this particular table in the logical layer.
The logical layer is the default view in the canvas and allows the user to relate or combine to other tables through a relationship (“noodles”).
- Any tables in this layer are called logical tables and can relate to other logical tables
- The level of detail is at the row level of the logical table
- Logical tables do not join physically and instead remain distinct (normalized)
- Requirements for relationships
- The ID used for joining must be the same data type
- Relationships can not be defined based on geographical fields or published data sources
- Circular relationships are not supported
- Limitations
- Dirty tables can make the analysis difficult
- Data source filters can limit Tableau’s ability in simplifying queries by removing unnecessary joins (join culling)
- Tables with a large amount of unmatched data
- Benefits
- Unmatched measure values are not dropped
- Avoids data duplication and filtering issues
- Generates queries based on the current view
The physical layer can be accessed by double clicking one of the logical tables. At this layer, the user can then make the standard joins and unions.
- This is called the Join/Union Canvas in the Data Source page
- Tables here are called physical tables and can be joined or unioned to other physical tables
- The level of detail is at the row level of the joined/unioned physical table (a single flat table that defines the logical table)
*Note: Tableau 2020.2 and earlier does not have the logical layer.
*A little information about the Data Source Page itself:
A.Left pane - Displays the connected data source and other details about your data.
B.Canvas: logical layer - The canvas opens with the logical layer, where you can create relationships between logical tables.
C.Canvas: physical layer - Double-click a table in the logical layer to go to the physical layer of the canvas, where you can add joins and unions between tables.
D.Data grid - Displays first 1,000 rows of the data contained in the Tableau data source.
E.Metadata grid - Displays the fields in your data source as rows
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