Since the Salesforce and MySQL data can not be joined in current versions of Tableau you will need to use a blend. To do this build out your report using the MySQL data then create a connection to the Salesforce data set. To define the key fields between the data sources go to Data>Edit Relationships>Custom and list out the key fields. Once you have done this drop the field from the Salesforce data set onto the filter shelf and you should be able to restrict the view as needed. There are likely many additional complications based on your data set and the view you are using but attached is a basic example of this using a blend between two separate files. I hope this helps!
Blend Filter.twbx 1.2 MB