You can achieve this using tabcmd. You need to install tabcmd on Tableau Server or any server that has access to Tableau Server and run the following command in Task Scheduler.
tabcmd refreshextracts --project "Sales External" --datasource sales_ds
As mentioned by a previous post TabCmd is an option for published data sources, however i dont believe there is a way to trigger one specific data source if you publish them embedded in the workbook. You can also achieve this by checking Include External Files when publishing. This will take a copy of your local files and push them to Tableau Server and then Tableau Server will use these pushed files when it refreshes. The other option would be to publish your SQL db as a published data source. This separates it from the other extracts in the workbook and allows it to be run individually.