SQL statement select columns with specific value - sql

Question

I need some help making an sql statement; I don't really know how to aproach the situation. I have two tables, Departments and Employees
from which I want to select the Dpt_num and the Dpt_name of the departments that have at least one employee and that all their employees are from Barcelona
Case 1
== Departments =======
| Dpt_num | Dpt_name |
| 1 | A |
| 2 | B |
== Employees ===================
| E_num | Dpt_num | City |
| 1 | 1 | Barcelona |
| 2 | 1 | Barcelona |
The result in this case should be
Dpt_num Dpt_name
------------------
1 A
Case 2
== Departments =======
| Dpt_num | Dpt_name |
| 1 | A |
| 2 | B |
== Employees ==================
| E_num | Dpt_num | City |
| 1 | 1 | Barcelona |
| 2 | 1 | Madrid |
The result in this case should be empty.
I tried this for example but it seems very inefficient and it does not work in all the cases
select
num_dpt, nom_dpt
from
departements
where
1 = (select count(distinct e.ciutat_empl)
from empleats e
where e.num_dpt = num_dpt)
and not exists (select * from empleats e
where e.ciutat_empl != 'BARCELONA' and e.num_dpt = num_dpt);
I really appreciate any help. Thanks!

Answer 1

You want to go down the path of doing the filtering in the where clause. Then, use exists and not exists:
select d.num_dpt, d.nom_dpt
from departaments d
where exists (select 1
from empleats e
where e.num_dpt = d.num_dpt and e.ciutat_empl = 'BARCELONA'
) and
not exists (select 1
from empleats e
where e.num_dpt = d.num_dpt and e.ciutat_empl <> 'BARCELONA'
);
The first condition checks that at least one employee is from Barcelona. The second checks that no employees are from any other city.
One major problem in your version is your correlation clause:
e.num_dpt = num_dpt
You think this is doing:
e.num_dpt = departaments.num_dpt
But it is really doing:
e.num_dpt = e.num_dpt
Always qualify your column names. This is especially important when you have more than one table reference in the query.

Answer 2

Join the tables, group by the department and check if the count of employees in Barcelona is equal to the count of all employess of the department.
SELECT d.dpt_num,
d.dpt_name
FROM departments d
INNER JOIN employees e
ON e.dpt_num = d.dpt_num
GROUP BY d.dpt_num,
d.dpt_name
HAVING count(CASE
WHEN e.city = 'Barcelona' THEN
1
END) = count(*);

Answer 3

I believe this should work:
select d.dpt_num, d.dpt_name
from departments d
inner join employees e on
d.dpt_num = e.dpt_num
group by d.dpt_num, d.dpt_name
having count(*) = sum(case when e.city = 'Barcelona' then 1 else 0 end)
INNER JOIN makes sure there's at least 1 employee
HAVING count(*) = sum(case when e.city = 'Barcelona' then 1 else 0 end) makes sure that all employees are from Barcelona

Answer 4

demo: db<>fiddle
SELECT dpt_num, dpt_name
FROM (
SELECT d.dpt_num, d.dpt_name, array_agg(city) as cities
FROM dept d
JOIN empl e
ON d.dpt_num = e.dpt_num
GROUP BY d.dpt_num, d.dpt_name
) s
WHERE 'Barcelona' = ALL(cities)
Aggregate the cities and then you can filter with the ALL operator which checks if all array elements fit the condition.

Answer 5

Generally speaking, you compare COUNT(*) with COUNT(some condition) for such problems:
SELECT *
FROM Departments
WHERE EXISTS (
SELECT 1
FROM Employees
WHERE Employees.Dpt_num = Departments.Dpt_num
HAVING COUNT(*) > 0 -- it is possible to get a 0 if where did not match
AND COUNT(*) = COUNT(CASE WHEN Employees.City = 'Barcelona' THEN 1 END)
)
DB Fiddle

Answer 6

Pl try query below
select a.dpt_number,a.dpt_name from yy_department a
where exists (select 'x' from yy_employees y where y.dpt_number = a.dpt_number and y.city = 'Barcelona')
and not exists (select 'x' from yy_employees y where y.dpt_number = a.dpt_number and nvl(y.city,'x') <> nvl('Barcelona','y'))