SQL: Output all names which belong to a category where all the countries within it are the same

Question

I have the following table called Stores:

Name          |   Country   |   Category 
Pharmacy          Japan         Health
Green Vine        Italy         Dining         


        

Answer 1

You can use EXISTS :

SELECT t.*
FROM table t
WHERE EXISTS (SELECT 1 
              FROM table t1 
              WHERE t1.Country = t.Country AND 
                    t1.category = t.category AND
                    t1.Name <> t.Name
             );

Answer 2

You can use following query to get all such categories for which all countries are same:

    Select t.category
    From table t
    Group by t.category
    Having count(distinct country)=1

Then you can use it to get StoreNames:

    Select t.name
    From table t
    Where t.category in
    ( Select t.category
    From table t
    Group by t.category
    Having count(distinct country)=1)

This syntax is compatible to MS sql server, but I hope logic can be translated to mysql.

Answer 3

You can use aggregation with a HAVING clause, that checks that the minimum country is the same as the maximum country for a category, i.e. all countries are the same (if there cannot be NULLs, which seems to be the case). Then inner join these categories.

SELECT *
       FROM elbat t1
            INNER JOIN (SELECT t2.category
                               FROM elbat t2
                               GROUP BY t2.category
                               HAVING max(t2.country) = min(t2.country)) x
                       ON x.category = t1.category;