If the data source of two tables is same then you can check this link: - Creating Joins | Tableau Software
You can also use: - Defining Custom Joins with the Custom SQL Option | Tableau Software
If you want to analyze the data from two different data source then you have to use data blending. In data blending, all the results are left joined on the common dimensions.
Hope this may help!
Thanks for they reply to my question. I do want to analyze the data from two different sources. For example I have a list of patients with unique patient id's and demographic information in dataset A. In data set B I have all visits to the doctor as individual rows, so patients that went to the doctor 5 times will have 5 rows of data. Each record has the patient id, so I want to do a one (dataset A) to many (Dataset B) join.
What I want to do is join the two together so that I can do things like show the average number of visits (dataset A) by gender (dataset B). What do you think is the best way to get the data linked in a way that I can do this type of summary?