Query to display specific columns using a Set operator - sql

Question

I am trying to use a Set operator to show country names(Table A col.) without a city(Table B col.) and cities(B) without a country(A). I have also tried to write this query using LEFT JOINS, which I show below and I included Table C(Regions) because I am not sure whether to use that primary key in a LEFT JOIN.
Table A (Countries):
Column_name | Column_id
|
COUNTRY_ID | 1
COUNTRY_NAME | 2
REGION_ID | 3
Table B (Locations):
Column_name | Column_id
|
LOCATION_ID | 1
CITY | 4
COUNTRY_ID | 6
Table C(Regions):
Column_name | Column_id
|
REGION_ID | 1
I have tried the following:
SELECT c.country_name, l.city
FROM Countries c
LEFT JOIN Locations l ON c.country_id = l.country_id
UNION
SELECT c2.country_name, l2.city
FROM Countries c2
LEFT JOIN Locations l2 ON c2.country_id = l2.country_id;
The SQL statement above returned all Table A values, and Table A values that do not contain Table B values (Countries that do not have Cities).
I also tried this statement below and got the exact same result:
SELECT c.country_name, l.city
FROM Countries c
LEFT JOIN Locations l ON c.country_id = l.country_id
LEFT JOIN Regions r ON r.region_id = c.region_id;
The one thing it is missing is Table A values not found in Table B (Countries not found in Cities.)

Answer 1

There are a lot of options to get your desired result. One way is to use LEFT JOIN to get the countries without city and RIGHT JOIN to get the cities without country:
SELECT c.country_name, l.city
FROM countries c
LEFT JOIN locations l ON c.country_id = l.country_id
WHERE l.city IS NULL
UNION ALL
SELECT c.country_name, l.city
FROM countries c
RIGHT JOIN locations l ON c.country_id = l.country_id
WHERE c.country_name IS NULL;
Another possibility is to use two LEFT JOIN, but starting from the opposite table, like this:
SELECT c.country_name, l.city
FROM countries c
LEFT JOIN locations l ON c.country_id = l.country_id
WHERE l.city IS NULL
UNION ALL
SELECT c.country_name, l.city
FROM locations l
LEFT JOIN countries c ON c.country_id = l.country_id
WHERE c.country_name IS NULL;
If you don't like using JOIN at all, you can do this using NOT IN:
SELECT c.country_name, NULL city
FROM countries c
WHERE country_id NOT IN (SELECT country_id FROM locations)
UNION ALL
SELECT NULL country_name, l.city
FROM locations l
WHERE country_id NOT IN (SELECT country_id FROM countries);
Or if you prefer NOT EXISTS, this will work, too:
SELECT c.country_name, NULL city
FROM countries c
WHERE NOT EXISTS (SELECT 1 FROM locations WHERE country_id = c.country_id)
UNION ALL
SELECT NULL country_name, l.city
FROM locations l
WHERE NOT EXISTS (SELECT 1 FROM countries WHERE country_id = l.country_id);
I created an example that shows all these queries will produce the identic outcome: db<>fiddle
Add ORDER BY c.country_name and ORDER BY l.city to the queries in case you want the result set to be sorted by them.
A last, but important note: As you see, I used UNION ALL instead of UNION because I don't see a reason why to use UNION in your use case. UNION ALL is much faster, so I recommend to use that unless there is a really convincing reason to do not use it. The only advantage of UNION is that it does not show duplicate rows, but I think they are very unlikely in your situation, so it should not be required.

Answer 2

The simplest illustration of using a set operator to find countries without cities would be:
select country_id from countries
minus
select country_id from locations
COUNTRY_ID
----------
1
4
As you need the country name, you just need to look it up:
select country_name from countries
minus
select c.country_name from locations l
join countries c on c.country_id = l.country_id;
COUNTRY_NAME
-----------------
England
Italy
Cities without a country (or with an invalid country code) is simpler as a left join and filter:
select l.city, l.country_id
from locations l
left join countries c on c.country_id = l.country_id
where c.country_id is null
CITY
-----------------
Berlin
Tokyo
If the requirement really is to do this using set operators, you would (conceptually) look for cities whose country_id is in the set of (location countries minus city countries):
select l.location_id, l.city, l.country_id
from locations l
where l.country_id in
( select country_id from locations
minus
select country_id from countries )
However this wouldn't give you locations whose country_id was null.