Numerical Value That Appears Several Times in Same Column - oracle-sqldeveloper

Question

So I have a table, Employees. I want to display the same month when employees were hired in department 50, 60, and 80. This is the code I wrote:
select to_char(hire_date, 'Month') as "Month Name"
from employees
where department_id = any (50, 60, 80);
However, this displays all the months when employees were hired in the three departments:
Month Name
January
May
February
November
October
January
March
July
January
May
March
Since January is the same month that appears in all three departments, I only want January to be displayed. How should I fix that?

Answer 1

You can count how many departments actually matched, which means you need to group by the month; and then filter on those that have the same number of matches as the number of departments you were looking for:
select to_char(hire_date, 'Month') as "Month Name"
from employees
where department_id = any (50, 60, 80)
group by to_char(hire_date, 'Month')
having count(distinct department_id) = 3;
Month Name
------------------------------------
January
February
May
That is looking for the same month in any year - which I guess might be useful to see who has anniversaries coming up. If you really want the same actual month, in the same year, then the grouping needs to include the year:
select to_char(trunc(hire_date, 'MM'), 'Month') as "Month Name"
from employees
where department_id = any (50, 60, 80)
group by trunc(hire_date, 'MM')
having count(distinct department_id) = 3;
Month Name
------------------------------------
February
This is with the standard HR schema's employee table; you may have different data in yours if you've been modifying it. In this data there are 12 people who are in one of those departments with a hire date in any February; three were hired in February 1999, one in each of those departments.