RE: How to identify/find the relation between parent and child data sources in queries
You say xpo, so you're using one of the older versions.
Apart from Cardinality (FetchMode), you first need to determine the relationship itself. If you look at the relationship tab of the child datasource, you can see through which filds the relationship is established.
In some cases, automatic relations is used. In these cases, you can see the relationship name in child ds.


In these cases, you need to go to the table of child ds and examine the relationship with the same name in order to see between fields.

Of course, in order to fully understand the relationship, you need to examine the concepts such as join type, cardinality, fetchmode(etc.).