Question

i am wondering why this fails

mysql> SELECT Continent C, Name, SurfaceArea
    -> FROM Country
    -> WHERE SurfaceArea = (
    -> SELECT MAX(SurfaceArea)
    -> FROM Country
    -> WHERE Continent = C);
ERROR 1054 (42S22): Unknown column 'C' in 'where clause'

its a answer provided by the certification guide for some sample exercises.

btw, for alias when do i have to use AS? isit optional?

Answer 1

In order to execute a correlated subquery, you need an alias for the outer table. You created an alias for the outer table’s field. Take a look at the corrected code below, that has an alias for the table (Cou) that is referenced in the subquery (note that the field alias is not required so I removed it. There’s no harm in adding it back if you wish):

SELECT Continent, Name, SurfaceArea
FROM Country Cou
WHERE SurfaceArea = 
(
    SELECT MAX(SurfaceArea)
    FROM Country
    WHERE Continent = Cou.Continent
);

Regarding the usage of AS, it’s optional. For example, in the query above you could write Country AS Cou, it would be the same.