Your Query

select ename from emp
where deptno = (select deptno from dept
where dname = 'Database');

Query Result

  ename
---------
 Suzuki
 Tanaka
 Matsuda
(3 rows)

Your Query

select ename from emp
where empno = (select manager from dept
where dname = 'Education');

Query Result

 ename
-------
 Satoh
(1 row)

Your Query

select dname, a.max from dept,
(select deptno, max(salary) from emp group by deptno) a
where dept.deptno = a.deptno;

Query Result

   dname   | max
-----------+-----
 Account   | 100
 Education | 150
 Personnel | 120
 Database  | 140
(4 rows)

Your Query

select  dname, a.avg from dept,
(select deptno, avg(salary) from emp group by deptno
 having count(*) >= 4) a
where dept.deptno = a.deptno;

Query Result

  dname  |         avg
---------+---------------------
 Account | 82.5000000000000000
(1 row)