The OUTER Join includes all rows in one table even if they have no matching values in the other. Either the LEFT of the RIGHT (or both) tables can be included in full. Typically we put the table to be shown in full on the LEFT
SELECT location, ename FROM dept LEFT OUTER JOIN emp ON dept.deptno = emp.deptno;
Boston is now included in the output, with the ename field NULL. Outer joins are often needed when workng with unknown data.