For a multiple libref join to pass to JDBC, all of these components of the LIBNAME statements must match exactly:
Outer joins between two or more tables can be passed to the DBMS. However, the outer joins must not be mixed with inner joins in a query.
PROC SQL attempts to pass joins across databases to the data source. However, the data source might or might not support this query. If the query cannot be completed in the data source, then the query is processed in SAS.
For more information about when and how SAS/ACCESS passes joins to the DBMS, see Passing Joins to the DBMS.