issues

can we over-ride sql over-ride at re-usable Source Qualifier at session level ?

Ans: You can not create re-usable source qualifier tranformation. If you have a sq tranformation, you can over-ride its sql query at session level.

Suppose you have un-connected lookup, having lookup override as below

SELECT DEPARTMENTSBKP.DEPARTMENT_NAME as DEPARTMENT_NAME, DEPARTMENTSBKP.DEPARTMENT_ID as DEPARTMENT_ID FROM DEPARTMENTSBKP
where DEPARTMENTSBKP.DEPARTMENT_ID = 20

You used this un-connected lookup at Mapping and there you changed the query to

SELECT DEPARTMENTSBKP.DEPARTMENT_NAME as DEPARTMENT_NAME, DEPARTMENTSBKP.DEPARTMENT_ID as DEPARTMENT_ID FROM DEPARTMENTSBKP
where DEPARTMENTSBKP.DEPARTMENT_ID = 10

When you run the session, always over-ridden query (that is dep = 10) will get executed. You can change the conditions, and some properties at mapping level of re-usable lookup. You can not change existing ports at mapping level. if you want to change the ports, you need to go back to Tranformation Developer.

In case of aggregator, if we do not select any port as a group by what will be the output? In my case only 1 rows got through...

What will happen if we do not use aggregate function and only 2-3 grup by ports, what is the output and how it works exactly ?

Please refer below comments for the answers of aggregation at Informatica.