如何在一列中选择具有相同值但在另一列中所有列必须彼此不同的行

Question

Hello everyone i will try to be as specific and precise as possible.

I was looking around for answer to my problem and in mean time i made 2 tables

In first table i have names of Companies and their id-s.

In second table i have names of Employees,their id-s,the cities in which they live,and idK-s(its supposed to match id id-s from previous table) of Companies they work in.

I am supposed to select names of Companies that have at least four(3 or more) employees living in different cities compared to each other.

In order for things to be more clear i am going to give an example:

table1:

id  name

1   Company1
2   Company2
3   Company3
4   Company4
5   Company5
6   Company6

table2:

id  name     city        idK

1   EMP1     city1       1
2   EMP2     city2       1
3   EMP3     city3       1
4   EMP4     city4       1

5   EMP5     city1       2
6   EMP6     city2       2
7   EMP7     city3       2

8   EMP8     city1       3
9   EMP9     city2       3
10  EMP10    city3       3

11  EMP11    city1       4
12  EMP12    city2       4
13  EMP13    city3       4
14  EMP14    city4       4
15  EMP15    city5       4

16  EMP16    city1       5
17  EMP17    city2       5
18  EMP18    city1       5
19  EMP19    city3       5
20  EMP20    city3       5   
21  EMP21    city2       5

22  EMP22    city1       6
23  EMP23    city8       6
24  EMP24    city1       6
25  EMP25    city15      6

I made spaces so it is easier to see. So in idK=1 we have four employees and all are from different city compared to each other so Company1 should be in results.

idK=2 and idK=3 have under four employees in general so Company2 and Company3 are out.

idK=4 is good same reasons as idK=1 but idK=5 and idK=6 are out because while they have four or more employees they are not from different cities compared to each other.

I found similar problems as mine and some solutions but none were good enough for me or i just couldn't make them work so i would be very grateful if someone could explain to me what to do in this situation.

Answer 1

You can use count(DISTINCT columnname) to get the number of unique values in a column.

SELECT
    c.`name`,
    COUNT(DISTINCT e.`city`) as `Num Cities`
FROM `companies` c
JOIN `employees` e
ON c.`id` = e.`idK`
WHERE COUNT(DISTINCT e.`city`) > 3
GROUP BY c.`name`
ORDER BY c.`name`

Answer 2

I expect this is what you are looking for

select idk
  from table2
 group by idk
having count(distinct city) > 3

Answer 3

This will work here.

select t1.name from table2 t2
inner join table1 t1 on t2.idK = t1.id
group by t2.idk,t1.name
having count(distinct city) > 3

This is the data I used to test on db-fiddle.

CREATE TABLE table2 (id int, name varchar(100), city varchar(100), idK int);


INSERT INTO table2 (id,name,city,idK)
VALUES (1,'EMP1','city1',1),
(2,'EMP2','city2',1),
(3,'EMP3','city3',1),
(4,'EMP4','city4',1),
(5,'EMP5','city1',2),
(6,'EMP6','city2',2),
(7,'EMP7','city3',2),
(8,'EMP8','city1',3),
(9,'EMP9','city2',3),
(10,'EMP10','city3',3),
(11,'EMP11','city1',4),
(12,'EMP12','city2',4),
(13,'EMP13','city3',4),
(14,'EMP14','city4',4),
(15,'EMP15','city5',4),
(16,'EMP16','city1',5),
(17,'EMP17','city2',5),
(18,'EMP18','city1',5),
(19,'EMP19','city3',5),
(20,'EMP20','city3',5), 
(21,'EMP21','city2',5),

(22,'EMP22','city1',6),
(23,'EMP23','city8',6),
(24,'EMP24','city1',6),
(25,'EMP25','city15',6);


Create TABLE table1 (id int,name varchar(100));

INSERT INTO table1 (id,name)
VALUES (1,'Company1'),
(2,'Company2'),
(3,'Company3'),
(4,'Company4'),
(5,'Company5'),
(6,'Company6');