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One cannot join across different data sources directly. It may be the case, however, that you can use data blending in Tableau to achieve a lot of things. You use one source as the primary data connection within a sheet (primary is determined by which connection is used first) and then you can use one or more connections as secondary, tertiary, etc. sources.
There is also a ton of great info in the documentation that can be found here: http://onlinehelp.tableausoftware.com/current/pro/online/en-us/multiple_connections.html
Hope this helps,
Maybe it would be worth importing the Excel sheet to a temp table on your SQL server and then joining.
However, if that's not a workable solution, you can bring both in as separate data sources and then filter using a common parameter.
it's worth reading this white paper, walks you through practical examples about how to leverage this Tableau capability.