PostgreSQL joined tables with null condition variable

I've a problem with my sql query in PostgreSQL while I'm trying to join a lot of tables. In section 'WHERE' I've a few conditions where I'm comparing some id values, e.g.

operator.agent_id = app.operator_id;

When every id ha开发者_运维问答s no null value, everything works, but in case even one id equals null, when query result is empty. Can I obtain such a result, that if one id equals null, then only values from this table are null, but rest is correct ?

To be more specific, I'm programming in Java, using EJB3:

stringQuery = "SELECT .... FROM ... WHERE...";
Query query = entityManager.createNativeQuery(stringQuery);
List<Object[]> objects = query.getResultList();

SELECT  *
FROM    app
LEFT JOIN
        operator
ON      operator.agent_id = app.operator_id

Move it to your join (left join) having the condition in your WHERE clause causes an inner join.