Ms sql server constraint key

For making constraint key in existing tables,two  tables are created authors and books. Here table authors is considered as parent table in which we have not defined any primary key column. And table books is taken as child table but foreign key column is not defined. 
create table authors(
id int  identity(1,1)
,FirstName varchar(100)
,Age int
)
create table books(
id int primary key  identity(1,1)
,Book varchar(100)
,AuthorID int
)
Creating Primary Key on existing column
Syntax : ALTER TABLE table_name ADD CONSTRAINT constraint_name PRIMARY KEY CLUSTERED (column(s) ASC | DESC)

Example: ALTER TABLE authors ADD CONSTRAINT  pk_author PRIMARY KEY 
CLUSTERED (id asc);

 Adding Default Constraint to  existing column
Syntax : ALTER TABLE table_name ADD CONSTRAINT constraint_name DEFAULT 'default_value' FOR column_name
Example :ALTER TABLE authors ADD CONSTRAINT def_age DEFULT '18' for Age;

 The table books  has column AuthorID as foreign key but  we have not defined foreign key. Now let us learn how to define foreign key constraint

Adding Foreign Key constraint
Syntax : ALTER TABLE table_name ADD CONSTRAINT constraint_name FOREIGN KEY(child_table_column_name)  REFERENCES parent_table (primary_key_column_name_of_parent_table) ON DELETE CASCADE ON UPDATE CASCADE
Example :
ALTER TABLE books ADD CONSTRAINT fk_books_author FOREIGN KEY(AuthorID)
REFERENCES authors(id)ON DELETE CASCADE ON UPDATE CASCADE