There are various use cases where we start out with a “flat” table like the Superstore sample data that has a number of columns with various dimensions and we want to make a simple list of unique values of one or more dimensions. such as a list that has just the six continents in Superstore:
The use cases for this include:
- Using a filter action value as a parameter in the target source (look for posts from myself and Rody Zakovich on this in the next week).
- Cross data source filters with higher performance when the list of filter values can be small compared to the volume of data.
- Creating scaffold data sources to pad out data and ensure there are no sparse combinations of values.
- Situations where we’d want to do a union or cross product of the data to do something like a market basket analysis but the union or cross product would be prohibitively large, so instead we only union or cross product desired dimension(s) and then join in the original data as necessary.
- The last multi-select highlighter method from Multiple Ways to Multi-Select and Highlight in Tableau can use a self-union.
If you are starting out with a well-structured data warehouse with dimension tables, can write SQL, Python, or R, build custom views on the data source, use data preparation tools like Alteryx or Easymorph or Trifacta, etc. then obtaining or generating these kinds of lists is pretty straightforward. But not everyone has those skills or resources, and in the case of users who just have Excel and/or text files we need to get creative. This post goes through a three different methods to get these lists in Tableau:
- Aggregated Extract
- Excel Pivot Table as a Data Source
- Custom SQL
In this post I’ll go through each of these options. [Note: this post was updated on 10 Jan 2018 to make the aggregated extract method a little simpler.]
This might seem obvious, but sometimes we’re stressed out and under deadlines and don’t realize we might be able to get help. If the data you are working with is coming from someone else then go ahead and ask them if they have a list of unique values. I’ve found that most people want the data they produce to be used and used well and if I’m coming back to them asking for something so I can do more with “their” data they are happy to accommodate me. I might phrase the request like “I want to make sure I’m using the latest list of departments, can you give me that list?”
The one caveat to getting data back from your ask is that you’ll need to go through some validation to make sure the list matches up with the “real” data, sometimes the amount of validation and cleansing isn’t worth the effort and one of these other approaches is better. However if you’re in a data-starved environment the kind of relationships you can make by asking for data can lead to more trust and ultimately more access to the data you want (and need).
2. Aggregated Extracts
For this method we’re going to connect to the data source and build an extract only we’ll be telling Tableau to aggregate the data to the desired level of detail (the field(s) we want to use) before Tableau builds the extract. The resulting extract then just has a record for each combination of field(s) that we want to use.
- Connect to the data source.
- Create a single worksheet with the field(s) you want to use as dimension pills, I usually just put them on Rows as discrete (blue) pills:
- Right-click on the source and choose Extract Data… The Extract Data window opens.
- Click on the Aggregate data for visible dimensions checkbox.
- Click the Hide All Unused Fields button.
- Click Extract. Tableau will ask where to save the extract. Choose a location and click OK.
Voila, you now have an aggregated extract source that you can use in Tableau data blends and/or join to!
Notes on Aggregated Extracts
There are a few things to keep in mind when using aggregated extracts: First of all there’s the need to refresh them to keep up with the data so if you have Tableau Server you’ll need to set up an appropriate schedule, if not then you’ll need to set up your own manual or automated workflow that gets the results you need. One possibility is using Tableau’s extract API.
Secondly if new columns are later added to the data they are automatically added to the extract. This may be ok for some use cases, there are others where this will break views that depend on that extracted data.
Finally, if you want to join on this aggregated extract you’ll need to join directly to the .tde or .hyper file. Where this gets complicated is handling data updates. You’ll need one workbook or workflow to update the extract and then use the extract in a second workbook. Unfortunately we can’t publish the extract to Tableau Server or Online and join to that published data source (yet), otherwise that would be an easy workaround. There are a number of cases where a Tableau data blend is sufficient, we’ll be demonstrating one in the next week.
3. Excel Pivot Table as a Data Source
For Excel sources besides connecting to worksheets with raw data we can connect to worksheets that are built as a pivot table.
- Open the source in Excel.
- Create a pivot table in a new worksheet.
- Drag the field(s) you are interested in to Rows.
- Rename the Row Labels header to have appropriate values if necessary.
- Remove the grand total.
- Rename the worksheet to something more meaningful than Sheet2.
- Save the workbook in Excel.
- Open up Tableau and connect to the Excel workbook.
- Drag the pivot table you just added onto the canvas:
Notes on using Excel Pivot Tables as a Data Source
Before Tableau introduced Level of Detail expressions in version 9 I used pivot tables in production views to pre-aggregate the data for some values and also to create tables I could join on to pad out the data so I could be sure to see records for every (person, office, metric) for every month. This method has one potentially major challenge around data updates, though, and that is that if we have data in worksheet A and a pivot table in worksheet B and we update the data in A (such as adding a new value that should appear in the pivot table B) that change won’t be reflected in the pivot table B until there is an explicit command in Excel to update the pivot table B and then save the workbook.
Even though we can tell Excel to do things like “Refresh data when opening file” this flag is only detected by Excel, not Tableau. Therefore to get updates to the data to be reflected in the pivot table the workflow has to include the steps to do a Data->Refresh All or open the pivot table worksheet before saving the workbook.
4. Custom SQL for Excel & Text Files
When I’m delivering Tableau training classes and we get to the point of talking about SQL & Tableau there are two common reactions: 1) yeay! and 2) [eyes glaze over]. This part is for the people in the latter category. Tableau hasn’t turned everything we might want to do into point & click, so sometimes we need to work with raw data. We do this in our everyday lives…there’s no good vegetarian restaurant in my town so when my wife & I want African ground nut stew we’ve got to make it ourselves. So I think of using Custom SQL as using the raw ingredients of the data to get a result I don’t have another way to get. However, in this case we’re going to be lazy (in a good way) and make Tableau write the SQL for us! Here’s how (these instructions don’t work for Tableau for Mac, see the Notes section below for more info):
- Start adding a new data source that is the Excel or text file you want to connect to.
- In the Open dialog select the file, then on the Open button click the drop down carat and choose “Open with Legacy Connection”. You’ll return to the data source window.
- Drag the worksheet or file if necessary onto the canvas.
- Use the Data->Convert to Custom SQL menu option. The Convert to Custom SQL window will appear.
- Edit the Custom SQL to remove all the fields that you don’t need.
- Make sure to delete the trailing comma from the last field in the SELECT before the FROM.
- Add the DISTINCT keyword after the SELECT before the first field. The SQL query will now look something like this:
- Click Preview Results… to test. If it comes back with an error then check your syntax (see notes below for some tips) and try again. If it works by showing a View Data window with your results close the View Data window and then click OK to close the Custom SQL window You’ve now created a unique list of values using custom SQL!
The advantage of using Custom SQL compared to using an aggregated extract or pivot table is that it updates with the data and doesn’t require the more complicated workflows of the other methods.
Simple SQL SELECT Query Syntax
SELECT DISTINCT [table].[field1] AS [field1] FROM [table]
SELECT DISTINCT [table].[field1] AS [field1], [table].[field2] AS [field2], [table].[field3] AS [field3] FROM [table]
In some ways SQL is written a little backwards, and in more complicated queries backwards and forwards. To me the real “starting place” of a SQL query is the FROM part because that is telling the SQL engine where (what table, worksheet, or text file, generically called “table”) to get the data from. Then the SELECT is going to grab the set of fields that we specify. The DISTINCT keyword tells the SQL engine to only get the unique (distinct) combinations of values of those fields instead of grabbing every single record.
The field names themselves use the [table name].[field name] convention so that if there are multiple tables in a query each field referenced can be uniquely identified. The table and field names are surrounded by square brackets by default to handle situations where the table or field name might have spaces. Finally Tableau uses the AS [field name] aliasing option to ensure that the name used by Tableau is a usable name in Tableau.
SQL doesn’t care about spaces & line feeds, we could write SELECT DISTINCT [table].[field1] AS [field1] FROM [table] all one one line and it would work just fine.
SQL cares very much about the placement of square brackets & commas, if one is out of place or missing then the whole query will fail. Make sure that you have all brackets in place and make sure that the last field in the SELECT doesn’t have a comma after it.
Notes on Custom SQL for Excel & Text Files
The Legacy Connector is not available on Tableau for Mac, so we can’t use this particular method for connecting to Excel or text files on the Mac.
The Legacy Connector is actually the Microsoft JET driver that was phased out in Tableau version 8.3 for a variety of reasons, here’s a link of differences to be aware of from the Tableau legacy connector documentation. Also here’s the Tableau documentation on Connect to a Custom SQL Query. Finally I did a post awhile back on details of using the Custom SQL in the context of Microsoft Access connections which also use the MS JET driver, some of the points there are useful to keep in mind.
If it all seems a bit hacky and contrived then I agree with you. At this time if all we have are Excel or text files and what features Tableau provides we’re in a low-resource environment and workarounds are necessary.
I regularly see projects I’m working with needing to invest more in data preparation in order to keep Tableau humming along. That investment could be in scripting languages like Python or PowerShell or R, using PowerQuery, starting the process of moving data into a database (there are free versions of many databases), and/or use more dedicated data preparation tools like Alteryx, Easymorph, or Trifacta. I like to set expectations around this early on in new projects because once they start using Tableau invariably projects run into imitations of their existing data pipeline to provide the volume and variety of data that they can now analyze in Tableau.
The goal for this post was to set you up with the skills you need to get a custom list of distinct values to support several different use cases and I hope this did that for you. As mentioned early on, Rody Zakovich and I have some posts in the works that use this to do some new things in Tableau!