I would talk to the ETL team to push either ReutersID into core or SystemID into RL to accomplish the task.
You can also create a view which is a combination of Market data, core & RL. That way, when Tableau imports it, they will all be seen as 1 table by Tableau. Does that help?
I dont see any other options to mingle core & rl without common fields
Thanks for your response however it is unlikely that I will be able to get any data source enriched so that the issue is no longer (especially as I don't have ownership over these data sources).
Are you sure that there is no way for Tableau to handle this and blend the data? After all, 1 data source does have a relationship to the others...
It appears that I have managed to get this to work via Tableau's edit Relationships. This was achieved by using 'Market data' as the Primary Data Source in relation to both 'Core' and 'RL', using 'SystemID' and 'ReutersID', respectively.
The next stage was to place the linked dimensions of 'Core' and 'RL' on either side of the related 'Market data' dimension' on the rows shelf.
The result is that I am now seeing data from each source on a row as long as it exists within the 'RL' data source.