Workaround for the Tableau “custom SQL” restriction on SQL Server variables

SQL Server (among other databases) has a handy feature for easy re-use of queries, in the guise of variables. You can declare variables and use them repeatedly in any query in the current session. That’s pretty handy for any complicated query forms you use repeatedly, where each time you might need to change some basic criteria.

As a trivial example: here’s how to select all the customers I have that live in postcode 12345

DECLARE @postcode varchar(5)
SET @postcode = '12345'

SELECT POSTAL_CODE, CUSTOMER_NO FROM [MY_CUSTOMERS] WHERE POSTAL_CODE = @postcode

I can then run it another day for postcode 23456 by just changing the second line to

SET @postcode = '23456'

This example is a bit silly because you might as well just type 12345 or 23456 into the query itself – but imagine a more complex SQL statement, or set of coordinated SQL statements, with screenfuls of code, and you might see how it makes it much easier to maintain.

But, sad to say, you can’t use these sort of variables as part of the custom SQL feature in Tableau. If you try, then you’ll get this slightly unhelpful message:

[Microsoft][SQL Server Native Client 11.0][SQL Server]Incorrect syntax near the keyword 'DECLARE'.
[Microsoft][SQL Server Native Client 11.0][SQL Server]Incorrect syntax near ')'.

But rather than have to hardcode your queries and modify them for every extract you can use a very simple alternative: our friend, the Tableau parameter.

For each SQL variable, create a Tableau parameter of the same data type. You can put in a default value as the “Current value” when setting it up.

Then, when connecting to your database, paste in the original SQL statement you had into the Custom SQL box, but removing the variable declaration and setting from the top.
SELECT POSTAL_CODE, CUSTOMER_NO FROM [MY_CUSTOMERS] WHERE POSTAL_CODE = @postcode

One at a time, highlight each SQL variable there, i.e. the @postcode part of the above, press the “Insert Parameter” button under the text field and select the appropriate Tableau parameter.

(There is actually the option to create a new parameter at this stage, if you didn’t do so in advance).

You’ll end up with a similar looking SQL statement, but with the variables now referred to in this sort of way:

SELECT POSTAL_CODE, CUSTOMER_NO FROM [MY_CUSTOMERS]
WHERE POSTAL_CODE = <Parameters.Postcode>

And it’ll work!

You can then use the standard Tableau functionality to change the parameters in Tableau, and behind the scenes your SQL statement will update appropriately.

Note firstly, for performance, that each change of this sort of parameter is going to cause the query to be re-run on the database (unless it’s already been cached).

Secondly, if you have created a Tableau Data Extract from this datasource, then altering the parameter won’t change anything until you refresh the extract.

Finally, the Tableau parameter is simply a normal Tableau parameter, just like anything else. So there’s nothing to stop a Tableau user altering it, which depending on your setup might be either exactly what you want, or exactly what you don’t want. If the latter, you might consider having the datasource involving parameters in a separate workbook to the user-facing dashboard (publishing to Tableau Server to share), or at least making sure it’s not visible on the end-user dashboards if they have no reason to change it.

Quick sidenote: I wondered if the “Initial SQL” feature might work, to declare variables upon connection to the datasource which could then be used in custom SQL later. But sadly not, I guess it doesn’t count as the same session. If you try, you’ll see a message like:
[Microsoft][SQL Server Native Client 11.0][SQL Server]Must declare the scalar variable "@postcode"