Postgres 13

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CREATE TABLE employee(
                        id serial PRIMARY KEY,
                        first_name varchar NOT NULL,
                        manager_id integer,
                        FOREIGN KEY(manager_id) REFERENCES employee(id)
                     );
INSERT INTO employee(first_name, manager_id)
              VALUES('Arden',          null),
                    ('Oliver',         null),
                    ('Luisa',          null),
                    ('Amelia',         null),
                    ('Olivia',         null),
                    ('Lily',              2),
                    ('Ava',               2),
                    ('Isabella',          2),
                    ('Charlie',           2),
                    ('Beatrice',          3),
                    ('Stephanie',         3),
                    ('Emily',             3),
                    ('Mila',              3),
                    ('Isla',              4),
                    ('Ashley',            4),
                    ('James',             7),
                    ('Jack',              7),
                    ('William',           8),
                    ('Harry',             8),
                    ('Robin',             8);

CREATE TABLE

INSERT 0 20

employee	highest_level_manager
Robin	Oliver
Harry	Oliver
William	Oliver
Jack	Oliver
James	Oliver
Ashley	Amelia
Isla	Amelia
Mila	Luisa
Emily	Luisa
Stephanie	Luisa
Beatrice	Luisa
Charlie	Oliver
Isabella	Oliver
Ava	Oliver
Lily	Oliver
Olivia	null
Amelia	null
Luisa	null
Oliver	null
Arden	null

SELECT 20