This happens in OOo 3.3, dev300m100, cws dba34d.

This happens, too, in libO 3.3 see bug report 34410: https://bugs.freedesktop.org/show_bug.cgi?id=34410

SELECT "Funktion"."Funktion", "Name"."Name", "Name"."ID" AS "MitgliedNr.", "rel_Fun_Nam"."Datum" 
FROM "Funktion" 
LEFT JOIN "rel_Fun_Nam" 
ON "rel_Fun_Nam"."funID" = "Funktion"."ID" AND "rel_Fun_Nam"."EndDat" IS NULL 
LEFT JOIN "Name" 
ON "rel_Fun_Nam"."namID" = "Name"."ID" 
ORDER BY "Funktion"."ID" ASC

To reproduce open odb file attached to the LibO bug.

Run query, the results contains,
 - 0  in  fields "MitgliedNr., 
 - 1-01-02 - wrong date - instead of null in both places.
 
If I run query in SQL direct mode - get same result.

If I run query in hsqldb manager, gives correct results.

If you open query for edit in dba34d cws I get assertion:
---------------------------
Debug Output
---------------------------
Error: Dialog::StartExecuteModal() - Parent not visible
From File D:/tools/o_dba34d/DEV300/ooo/vcl/source/window/dialog.cxx at Line 636
Abort ? (Yes=abort / No=ignore / Cancel=core dump)
---------------------------
Igen   Nem   Mégse   
---------------------------

When click on no, then opens in SQL view.

If you try to switch from SQL view to design view, I get error:

SQL Status: HY000
Error code: 1000

Join could not be processed