Which database? The source database? It can have lots of primary keys. Every table can have a primary key. Or do you mean the in-memory database of the Qlik Sense app? That doesn't have primary keys. It will have fields marked as keys which means those fields are used to associate tables with each other.
Qliksense just loads the table from the databases. It do not have any primary keys in built, there is only key fields in Qliksense, that is the join fields between the two tables based on the column names. There is no concept of Primary key or Foreign key in Qlikview.
Thank you for the aswers. That's because I'm dealing with a DB that I do not have the data dictionary, in other words, I would like to link tables just how they are linked in the source database. So I thought if the "primary key" gave by Qlik Sense was the primary key of the source.