Abap and SQL Request (BKPF / BSEG)

Hello, I'm beginner in Abap and I have a question about a query on the table BKPF and BSEG.

I need the list of accounting documents (BKPF) which have accounting entry for a project given in parameter (Field Projk of table BSEG).

Since it's impossible to make a "JOIN" with tables BKPF and BSEG, how can I write my SQL query ?