Getting common values in a table based on a subquery - mysql

Question

I couldn't find what I am looking for even though it seems like a 'simple' thing to do in SQL.
So here it is,
I have a table containing dates and countries, let's call it A.
I have a subquery that selects a list of countries from a table C, let's call it B.
The goal is to return only and distinctively the dates from A that have every single country in B
For example,
A B (SELECT FROM C WHERE ...)
date country country
2020-07-21 1 1
2020-07-21 2 2
2020-07-12 1
2020-07-12 2
2020-07-06 1
2020-07-06 2
2020-07-06 3
Should return
date
2020-07-21
2020-07-12
2020-07-06
But if B was
B
country
1
2
3
then it should return
2020-07-06
The whole point is to get all the dates that have all the country listed in the B list
I tried
SELECT DISTINCT T.date FROM (
SELECT date
FROM A
WHERE country = ALL (SELECT country FROM C WHERE ...)
) AS T
But it doesn't return anything because I think it only returns the list if it is true for everything.
Here is an MCRE:
/*!40101 SET #saved_cs_client = ##character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `countries` (
`country_id` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`country_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = #saved_cs_client */;
/*!40101 SET #saved_cs_client = ##character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `projects` (
`project_id` int(11) NOT NULL AUTO_INCREMENT,
`project_date` date NOT NULL,
`country_id` int(11) NOT NULL,
PRIMARY KEY (`project_id`),
KEY `country_id` (`country_id`),
CONSTRAINT `projects_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `countries` (`country_id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = #saved_cs_client */;
INSERT INTO countries VALUES (1);
INSERT INTO countries VALUES (2);
INSERT INTO countries VALUES (3);
INSERT INTO projects(project_date, country_id) VALUES ('2020-07-21', 1);
INSERT INTO projects(project_date, country_id) VALUES ('2020-07-21', 2);
INSERT INTO projects(project_date, country_id) VALUES ('2020-07-12', 1)
INSERT INTO projects(project_date, country_id) VALUES ('2020-07-12', 2)
INSERT INTO projects(project_date, country_id) VALUES ('2020-07-06', 1);
INSERT INTO projects(project_date, country_id) VALUES ('2020-07-06', 2);
INSERT INTO projects(project_date, country_id) VALUES ('2020-07-06', 3);
Then this is what I try to do
SELECT project_id, project_date
from projects
where country_id = all (select country_id from countries where country_id in (1,2))
group by project_date
I know the subquery is useless here but in reality, the condition is different from IN (1,2) but this simplifies it without losing the meaning of what I'm trying to do.
In this database, with (1,2) in the subquery, it should return every date as they all have the country_id 1 and 2. However, if it was set to (1,2,3) it should only return '2020-07-06' as a group because it is the only date with the country_id 1,2 and 3

Answer 1

Assuming that your query that returns the countries from C is something like:
select country_id from countries where country_id in (....)
then use it like this:
select project_date
from projects
where country_id in (select country_id from countries where country_id in (...))
group by project_date
having count(*) = (select count(*) from countries where country_id in (...))
If there are duplicate countries in projects then change the HAVING clause to:
having count(distinct country_id) = (select count(*) from countries where country_id in (...))
See the demo.

Answer 2

You can try the below way -
select date
from tablename
group by date
having count(distinct country)=(select count(distinct country) from countryTable)