SQL Server 2022

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CREATE TABLE student (
    id   INT PRIMARY KEY,
    name VARCHAR(100) NOT NULL
);

CREATE TABLE classes (
    id   INT IDENTITY PRIMARY KEY,
    student_id INT NOT NULL,
    class_name VARCHAR(100) NOT NULL,
    FOREIGN KEY (student_id) REFERENCES student(id)
);

INSERT INTO student (id, name)
VALUES (123, 'abc'),
       (200, 'john');   -- extra student to show multi-row behavior

INSERT INTO classes (student_id, class_name)
VALUES (123, 'xyx'),
       (123, 'math'),
       (200, 'history');
5 rows affected

SELECT
    id,
    name,
    (SELECT class_name
     FROM classes c
     WHERE c.student_id = s.id
     FOR XML PATH('student_class'), ROOT('student_classes'), TYPE) AS student
FROM 
    student s
FOR XML PATH('user'), ROOT('users');


XML_F52E2B61-18A1-11d1-B105-00805F49916B


<users><user><id>123</id><name>abc</name><student><student_classes><student_class><class_name>xyx</class_name></student_class><student_class><class_name>math</class_name></student_class></student_classes></student></user><user><id>200</id><name>john</name><student><student_classes><student_class><class_name>history</class_name></student_class></student_classes></student></user></users>


SELECT
    id,
    name,
    (SELECT class_name
     FROM classes c
     WHERE c.student_id = s.id
     FOR XML PATH('student_class'), TYPE) AS [student/student_classes]
FROM 
    student s
FOR XML PATH('user'), ROOT('users');


XML_F52E2B61-18A1-11d1-B105-00805F49916B


<users><user><id>123</id><name>abc</name><student><student_classes><student_class><class_name>xyx</class_name></student_class><student_class><class_name>math</class_name></student_class></student_classes></student></user><user><id>200</id><name>john</name><student><student_classes><student_class><class_name>history</class_name></student_class></student_classes></student></user></users>