It is not possible to combine two tables with different sources,so first convert the department table normal oracle target

Mapping 1:

1: For cobol dept source use normalizer to convert to oracle target
2: For flat files simply convert to oracle using source transformer

Mapping 2:

1: Use the converted dept mapping 1 (oracle source) and emp table (oracle source) connect there source transformer to joiner tranformer using emp.deptno = dept.depno
2: Since joiner doesn't join directly with aggregator take the output to expression transformation then to aggregator apply max(sal) group by deptno
3. Finally take the output to target

Workflow:

Design the worflow(W1: Mapping 1) then workflow (W2: Mapping2),make w2 to run only if w1 is finished and successfully completed.