How my simple sub-query work when a aggregate function and a Group by clause is used?

Question

I have homework problem sorry I am very noob in this.

I have an Employee table. I want a Query to find the Name of an employee drawing highest salary in their departments

I have written:

Select emp_name 
from employee 
where salary=any(select max(salary) from employee group by dept_no)

Table Name: employee

emp_name  salary   Dept_no
e1         1000     10
e2         2000     10
e3         2000     20
e4         3000     20

output should be:

e2 
e4

but it is wrong as can someone tell me why?

Answer 1

Your code is missing the link of the departments and sets the condition only for salaries.
You must join your subquery to the table:

Select e.emp_name 
from employee e inner join(
  select dept_no, max(salary) salary
  from employee 
  group by dept_no
) t on t.dept_no = e.dept_no and t.salary = e.salary

Or with NOT EXISTS:

Select e.emp_name 
from employee e 
where not exists(
  select 1 from employee
  where dept_no = e.dept_no and salary > e.salary
) 

Answer 2

Your issue that that there is no connection between the departments. So, someone in Department B might earn the maximum salary of someone in Department A -- but might not be the highest earner in that department.

For this approach, I would recommend a correlation clause in the subquery, rather than GROUP BY :

select e.emp_name 
from employee e
where e.salary = (select max(e2.salary)
                  from employee e2
                  where e2.dept_no = e.dept_no
------------------------^ correlation clause
                 );

Note the use of table aliases and qualified column names. You should always qualify all table references when writing queries -- it ensures that the query does what you intend and it makes the query more understandable for others.

If performance is an issue, then use employee(dept_no, salary) .