As alias "cannot be used in case, only displays results when"

1. correct SQL

SELECT
    Cntry_name,
    cntry_continent as continent, case
    -Note: The field used for the case is the true field name of the table cntry_continent ("not an alias Continent "Oh, pro." When
        cntry_continent is null THEN ' continent are null '
        ELSE cntry_continent end as
    marvin_case _when_then_else_end--The column name for this column is: Marvin_case_when_then_else_end from
    EI. COUNTRIES;

2, error SQL

SELECT
    Cntry_name,
    cntry_continent as continent, case
    -Note: The field in the case is alias continent (this is not true, error execution)
        When continent are null THEN ' continent is null '
        ELSE cntry_continent-  Note: Cntry_continent is the true field name of the table, which is right (here Cannot use continent) end as
    marvin_case_when_then_else_end
from
    EI. COUNTRIES;

To post the execution results of the correct SQL