I cannot join tables within internal data source from Zoho CRM

I cannot join tables within internal data source from Zoho CRM

Hello there,

We have been sync'ing zoho crm and zoho analytics for a while. In zoho CRM, "Purchase Orders" and "Purchased Items" are 2 related tables on a same field, so I can export data from zoho crm, like 1 Purchase Order linking to 5 purchased items.

In zoho analytics, I firstly examined that all data fields from both "Purchase Orders" and "Purchased Items" are selected in the data sync set up. However, there is no connection/same field if I want to left join 2 tables. 

Can anyone help please? Thanks!