Oracle Sub-Query

SELECT * FROM teacher

--Name of all male teachers in admissions department
Is there an admissions department in the--teacher table? Some numbers!
--01. Query the Admissions Department for the corresponding number
Select Dname,deptno from dept where dname= ' admissions Department '
--02. So as long as the department number is 20, the admissions department
Select Tname,deptno
From teacher
where deptno= (select Deptno from dept where dname= ' Admissions Department ')

--All teachers ' names, departments
--How to convert Deptno into dname
Select Tname,deptno from teacher
Select Dname from Dept
--Replace the DEPTNO with the above statement
--sub-query as column name
Select Tname, (select Dname from dept where Deptno=t.deptno) as department
From teacher T

--a list of all teachers in the admissions department and the Human Resources department
Select Deptno,tname from
Teacher
where Deptno in
(select Deptno from dept where Dname in (' Admissions ', ' Ministry of Manpower ')

--use exists to check if the subquery returns a data performance higher than in
--use exists instead of in instead of exists in
--exists does not return actual data but returns TRUE or False
Select Deptno,tname from
Teacher where exists
(Select Deptno from dept
where
Deptno=teacher.deptno and Dname in (' Admissions ', ' Ministry of Manpower ')

Oracle Sub-Query