Show values from list that are NOT returned by query
You can use the VALUES clause to create a "constant table":

Let's assume this is your table:

CREATE TABLE some_table
(
    appropriate_column TEXT
) ;


populated with:

INSERT INTO 
    some_table (appropriate_column)
VALUES 
     ('something_b'),
     ('something_d') ;


This is how you would make the query you're looking for:



SELECT 
    v 
FROM
    (
    VALUES
        ('something_a'),
        ('something_b'),
        ('something_c'),
        ('something_d')
    ) AS vs(v)
WHERE
    v NOT IN 
    (SELECT 
        appropriate_column 
    FROM
        some_table
    ) ;


You can check everything at dbfiddle