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All Tableau Help > Tableau Help > 
Applies to: Tableau Desktop

Troubleshoot Data Blending

In this article

Common warnings and errors when blending data sources

Asterisks show in the sheet

Null values appear after blending data sources

Blending issues after publishing data sources

Blending with a cube (multidimensional) data source

Other data blending issues

When you use data blending to combine your data from different data sources, there are a few common issues that might arise. This topic lists them and describes how you can respond to each situation.

Common warnings and errors when blending data sources

No relationship to the primary data source

When you drag a field from a secondary data source to the view, you might see a warning that says:

Fields cannot be used from the [name of secondary data source] data source, because there is no relationship to the primary data source. In the Data pane, switch to the [name of secondary data source] data source, and click at least one link icon to blend these data sources.

This warning occurs when you have no active links in the secondary data source

For example, suppose you have two data sources that are related by the State and Date fields. At least one of these fields must have the active link icon () next to it in the secondary data source. You can make the link active by clicking the link icon in the Data pane or by using the related field from the primary data source in the view.

The secondary data source may not have any relationships to the primary data source. Look in the Data pane for the link icon. Tableau automatically links fields that have the same name. If your fields do not have the same name, you must define a relationship between them. For more information, see Step 4: (Optional) Define or edit relationships .

Primary and secondary connections are from tables in the same data source

When you drag a field from a secondary data source to the view, you might see a warning that says:

The primary and secondary connections are from tables in the same data source. Instead of linking the connections, use the Data menu to join the data. Joins can integrate data from many tables and may improve performance and filtering.

This warning occurs when the workbook contains separate data sources that connect to the same database. Though you can combine data in this way, Tableau recommends that you use a join to combine data from the same database instead. Joins are typically handled by the database, which means that joins leverage some of the database's native performance capabilities.

Cannot blend the secondary data source because one or more fields use an unsupported aggregation

Data blending has some limitations regarding non-additive aggregates such as COUNTD, MEDIAN, and RAWSQLAGG. Non-additive aggregates are aggregate functions that produce results that cannot be aggregated along a dimension. Instead, the values have to be calculated individually.

These limitations cause certain fields in the view to become invalid under certain circumstances. If you hover your mouse cursor over one of these invalid fields, you see the following error:

Cannot blend the secondary data source because one or more fields use an unsupported aggregation.

 

This error can occur for one of the following reasons:

 

Asterisks show in the sheet

When you blend data, make sure that there is only one matching value in the secondary data source for each mark in the primary data source. If there are multiple matching values, you see an asterisk in the view that results after you blend data.

For example, suppose you have two data sources: Population and Superstore. The primary data source, Population, has a field called State. The secondary data source, Superstore, contains fields called State and Segment. In the secondary data source, each state has multiple segments. Alabama, for example, has three segments: Consumer, Corporate, and Home Office.

Primary data source

Secondary data source

When you blend the two data sources on the State field, you create a relationship where individual state values (in the primary data source) can have multiple segment values (in the secondary data source). In this case, multiple values for segments in the secondary data source for each corresponding state value in the primary data source cause asterisks to appear in the view, as demonstrated by the images below.

Blended data

Resolve asterisks in the sheet

Avoid asterisks in the sheet by making sure that there is only one matching value in the secondary data source for each mark in the primary data source. Though the way you ensure this depends on the data and the view you're trying to create, consider one of the following suggestions to resolve asterisks in the sheet.

Null values appear after blending data sources

Null values can sometimes appear in place of the data you want in the view when you use data blending. Null values can appears for a few reasons:

Data blending works by supplementing the data in the primary data source with data from the secondary data source based on the linking field. This means Tableau takes all the data in the primary data source, and only the corresponding matches in the secondary data source. If Tableau cannot identify at least one matching value in the secondary data source for the corresponding value in the primary data source, null values appear.

You can resolve this issue by doing the following:

Blending issues after publishing data sources

When you make a field-level customization that blends two data sources, the customization is based on one of the data sources, the primary data source. Then, when the primary and secondary data sources are published to Tableau Server or Tableau Online, the primary data source, which contains the customization (i.e., the reference to the secondary data source) no longer works. The customization no longer works because the relationship between the primary and secondary data sources is lost. This is because the relationship between the primary and secondary data sources is defined at the workbook level.

For example, suppose you have two data sources: Store - Main and Store - West. You create a field-level customization, like a calculation, that combines the sales totals from the primary data source, Store - Main, with the sales totals from the secondary data source, Store - West. As soon as you publish the data sources to Tableau Server or Tableau Online, the calculation in Store - Main no longer works. This is because the calculation refers to the secondary data source, Store - West, whose location is now undetermined.

To work around this scenario, do the following:

  1. Before creating any field-level customizations, publish each data source first.

  2. Connect to each data source.

  3. Create a workbook that contains the field-level customizations that you need.

  4. Publish the workbook to Tableau Server.

    You can use this workbook as a "template" that can be shared and downloaded by you and others.

Blending with a cube (multidimensional) data source

Cube data sources can only be used as the primary data source for blending data in Tableau. They cannot be used as secondary data sources.

Other data blending issues

For other issues that emerge while blending data, see the following links.