What is the relationship between the two tables? What dimension are you joining them on? Right now you are doing a full join, which means that you are getting the rows with matched dimension values (inner part of the venndiagram) and the outer parts, which will have null values for the records from the other table.
Does that make sense? It looks like a basic joining problem, but that being said, I don't know your data.
My guess is that you are not joining on "Month" which looks to be the common dimension between the 2 tables. This is because you are getting nulls in the Month field for buyincome table.
So in this case should I create a foreign key between the months to link them in the Oracle database where the data comes from or is there some way to sort this out on Tableau?
I would start with:
- Only bringing the Table MONTHLYBUYINCOME in the data pane and checking if Tableau can display the data.
- Refreshing the extract
- Trying a live connection if you're Database is not too big and generating the extract later in the workbook
If it's ok have a look at this Tableau video to understand the different join types:
>You should use a Left or Right Join on the dimension you're joining on. In your case: Month.
If you left/ right join the tables it will return only the rows that exactly match the dimension you're joining on. So you should get only the months that are in both table and not null.