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 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;