I get an error when I run my query even though the syntax is corret : I have a table named CUSTOMERS
I have a column inside the table named CITY 
I want to retrieve the city that has the most customers, in other words the most frequent CITY in table CUSTOMERS.

I get an error message as following: ORA-00904: "COUNTRY": invalid identifier


    SELECT COUNTRY,
    COUNT(COUNTRY) AS `value_occurrence` 
    FROM CUSTOMERS
    GROUP BY COUNTRY
    ORDER BY `value_occurrence` DESC
    LIMIT 1;