SQL DML Foreign Key Constraint

CREATE TABLE Authors
(
AuthorId int PRIMARY KEY NOT NULL,
AuthorName varchar(100),
City varchar(50)
);

CREATE TABLE Books
(
BookId int PRIMARY KEY NOT NULL,
Title varchar(200),
AuthorId int,
FOREIGN KEY (AuthorId) REFERENCES Authors(AuthorId)
);

INSERT into Authors
(AuthorId, AuthorName, City)
Values
(101,'Mukharjee', 'Delhi'),
(102,'Benerjee', 'Culcutta'),
(103,'Netaji','Patna')

SELECT * FROM Authors

INSERT into Authors
(AuthorId, AuthorName, City)
Values
(104,'Robin', 'Hyderbad')

INSERT into Books
(BookId, Title, AuthorId)
Values
(1001, 'Passage to India', 103)

select * from books

INSERT into Books
(BookId, Title, AuthorId)
Values
(1002, 'Passage to USA', NULL)

INSERT into Books
(BookId, Title, AuthorId)
Values
(1003, 'Passage to UK', NULL)

SELECT * FROM Authors

UPDATE Books
Set AuthorId = 104
Where BookId = 1002

SELECT b.*,a.* FROM Books b left join Authors a on b.AuthorId = a.AuthorId

SELECT a.*, b.* FROM Authors a left join Books b on a.AuthorId = b.AuthorId

INSERT into Books
(BookId, Title, AuthorId)
Values
(1004, 'Passage to Bangladesh', 103)