MySQL 8.0

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CREATE TABLE Author (code VARCHAR(2), Name VARCHAR(9));

INSERT INTO Author (code, Name) VALUES

('GB', 'George B.'),

('KL', 'Kyle L.'),

('DC', 'Donald C.'),

('RL', 'Roland L.');

CREATE TABLE Book (isbn INTEGER, author VARCHAR(2));

INSERT INTO Book (isbn, author) VALUES

('1111', 'GB'),

('2222', 'DC'),

('3333', 'RL');

CREATE TABLE Coauthor (name VARCHAR(2), isbn INTEGER);

INSERT INTO Coauthor (name, isbn) VALUES

('KL', '1111'),

('GB', '2222'),

('GB', '3333');

SELECT a.code, a.name,

COUNT(DISTINCT b.isbn) + COUNT(DISTINCT c.isbn) count

FROM Author a

LEFT JOIN Book b ON b.author = a.code

LEFT JOIN Coauthor c ON c.name = a.code

GROUP BY a.code, a.name;