4 Replies Latest reply on Jun 12, 2018 4:39 PM by Anthony Cua

    Unexpected NULLs after Data Blending

    Anthony Cua

      Hi everyone,


      Need your help in looking at my table and dataset.

      I'm basically blending 2 datasets. Sales and Audience. If you look at the "sales" tab, I have 15483 for Brand A. 15490 is my expected number though.

      Switching the primary and secondary datasets, which I did in the "Audience" tab, reveals that there were 7 nulls. Which I don't understand why.


      Can anyone explain why Tableau is seeing nulls in the data? (I do need all 6 dimensions in the sheet as filters/linked fields.)




        • 1. Re: Unexpected NULLs after Data Blending
          Kawaljeet kaur



          It is because while you are switching data sets, whatever column you drag first on the shelf's acts as a primary data source and performs left join by default so in this case all the rows in the left table and matching rows in the right table i.e secondary data source will be blend.


          In your case while you are filtering there might be some unmatched columns that is why null has been shown.


          Please look into the below video on Data blending.





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          • 2. Re: Unexpected NULLs after Data Blending
            Anthony Cua

            Thank you Kawaljeet. I'm still confused why my data is showing as such though. It seems that Tableau is assuming certain values because of the amount of linking fields that I have (total of 6).


            I've decided to approach a solution without linking the the 2 datasets.

            • 3. Re: Unexpected NULLs after Data Blending
              Yuriy Fal

              Hi Anthony,


              There are certain combinations of [Audr] & [Region] & [Div Own] values

              in your Audience datasource that don't exist in your Sales data.


              For the Marks on a view drawn from that combinations of values

              Tableau would calculate the Null value for any Pill from the Secondary

              (and make the Null visible if such a Pill is on a view, as on your Audience sheet).


              If all links in the Secondary are in the Inactive state (as on your Audience 2 sheet),

              Tableau wouldn't do a Blend -- so there would be no Marks with the 'suspicious' Nulls.

              That's how Data Blending works, no surprise here.


              As for Cross-Datasource Filters, though they share common properties / conventions 

              (field names, data types and case-sensitive values should be the same in both datasources)

              & visual cues (chain links & orange checkmarks when filters are from Secondary),

              they are not the same as 'regular' Blended Filters from Secondary (their behaviour differ).


              Think of Cross-DS Filters as ones that are 'permanently linked' -- on every view where they're applied.

              And it is the actual Data Blending -- when fields from Secondary are present on a view, either as Pills

              on Rows / Columns / Details (but not Pages) shelves, or when referred in calculations -- that would complicate matters.




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              • 4. Re: Unexpected NULLs after Data Blending
                Anthony Cua

                That is a lot of info. I definitely appreciate you taking a look at my data and detailing the behavior for me Yuriy.


                I'll chew this more and will reply should I have any other questions. Again, I really appreciate this kind sir.