Add support for custom SQL

[benlower]

Enable modification of custom SQL in data sources

/cc @DataRoberts

[MikeRobertsPS]

This is great for when one has sql that changes frequently. So, for example, if you have a custom sql source on Github then the sql from the tableau data source/wb can also be updated in any number of workbooks.

[MikeRobertsPS]

Here's an example: https://github.com/DataRoberts/Tableau/blob/master/AnalyticsEngineering/UpdateCustomSQL.ps1

[bryanthowell-tableau]

[https://tableauandbehold.com/2016/06/29/defining-a-tableau-data-source-programmatically/] goes into what is necessary here. Custom SQL is just another relation, so it is tied to being able to create / modify the relations.

[florent-guinard]

Hi,

We'd also like to be able to update the Custom SQL of our data sources using the Document API. I saw that it is possible to retrieve the Custom SQL but it is not possible to update it, do you have this enhancement in your roadmap ?

Thanks !