1 Reply Latest reply on Sep 15, 2016 10:37 AM by diego.medrano

    Troubleshoot Data Blending-Common errors while Data Blending data source.

    CHENNA YENUGA

      Troubleshoot Data Blending

      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.

       

      Common warnings and errors when blending data sources

      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.

       

       

      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:

      • Groups in the primary data source: If a non-additive aggregation from the primary data source is in the view, you cannot use a group created in the primary data source. To work around this limitation, convert the group to a calculated field.
      • Non-additive aggregates from the primary data source: Non-additive aggregates are only supported in the primary data source if the data in the data source comes from a relational database that allows the use of temporary tables. To work around this issue, consider creating an extract of your data source. Extracts support temporary tables.
      • Non-additive aggregates from a multi-connection data source that uses a live connection: Multi-connection data sources that connect to data using a live connection do not support temporary tables. Therefore, using a multi-connection data source that connects to data using a live connection prohibits the use of blending functionality with non-additive aggregates. To work around this issue, consider creating an extract of your multi-connection data source. Extracts support temporary tables.
      • Linking field in the view before the use of an LOD expression: This error can also appear when you use a level of detail expression in a view that uses data blending. To resolve the error, make sure the linking field in the primary data source is in the view before you use an LOD expression from the secondary data source.
      • Published data sources as the primary data source:Because certain versions of Tableau Server does not support temporary tables, there are some limitations around non-additive aggregates.
        • For Tableau 8.3 and earlier, Tableau Server does not support temporary tables. Therefore, using a published data source as your primary data source prohibits the use of the blending functionality with non-additive aggregates.
        • For Tableau 9.0 and later, you can use COUNTD and MEDIAN with blending functionality in a published data source that is used as the primary data source. However, the other limitations listed above still apply.

       

      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 Consumer, Corporate, and Home Office segments.

      Primary data sourceSecondary data source

      When you blend the two data sources on the State field, you create a relationship where individual state values can have multiple segment values. 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.Avoid this by making sure that there is only one matching value in the secondary data source for each mark in the primary data source.Blended data

      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:

      • The secondary data source does not contain values for the corresponding values in the primary data source.
      • The data types of the fields you are blending on are different.
      • The values in the primary and secondary data sources use different casing.

      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:

      • Insert data in the secondary data source: Insert missing data in the secondary data source so that all records in the primary data source have a match. For example, suppose your primary data source contains values for all 50 U.S. states, but your secondary data source only has values for 30 U.S. states. To resolve this issue, you can add values for the remaining 20 U.S. states to the underlying data set of the secondary data source. Alternatively, you might be able to Bring a Field into the Primary Data Source or Alias Field Values Using Data Blending to work around null values if you cannot modify the underlying data.
      • Verify data types in the primary and secondary data sources match: For example, both data sources should use string data for the fields you are blending on, or both fields should contain numbers. You can't blend fields where one field contains numbers and the other contains strings.
      • Verify the casing of the values in the primary and secondary data sources match: If one data source uses a mix of upper- and lowercase names, and the other uses only uppercase, you'll see null values. For more information, see Nulls Show When Data Blending.

      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.

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