SQL Server 2017

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CREATE TABLE Teachers
(
  TeacherID INT PRIMARY KEY NOT NULL,
  TeacherName VARCHAR(45) NOT NULL
);

CREATE TABLE Classes
(
  ClassID INT PRIMARY KEY NOT NULL,
  Class VARCHAR(45) NOT NULL,
  MainTeacher INT NOT NULL,
  CONSTRAINT FK_Classes_MainTeacher FOREIGN KEY (MainTeacher) REFERENCES Teachers (TeacherID)
);
INSERT INTO Teachers (TeacherID, TeacherName) VALUES
(1, 'Teacher1'),
(2, 'Teacher2');

INSERT INTO Classes (ClassID, Class, MainTeacher) VALUES
(1, 'Class1', 1),
(2, 'Class2', 1),
(3, 'Class3', 2);
5 rows affected

SELECT T.*,
       STUFF(
               (
                 SELECT DISTINCT ',' + Class
                 FROM Classes C
                 WHERE C.MainTeacher = T.TeacherID
                 FOR XML PATH('')
               ), 1, 1, ''
            ) ControlledClasses
FROM Teachers T 


TeacherID
TeacherName
ControlledClasses


1
Teacher1
Class1,Class2

2
Teacher2
Class3