A problem usually encountered when writing sql, record it

There are three tables student (student table), teacher (teacher table), stu_teacher (student-teacher relationship table)

The table structure and data are as follows

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

INSERT INTO `student` VALUES ('1','Zhang San');
 INSERT INTO `student` VALUES ('2',' ');
 INSERT INTO `student` VALUES ('3',' ');
 INSERT INTO `student` VALUES ('4','Zhao Liu');

CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

 INSERT INTO `teacher` VALUES ('1','Chinese teacher');
 INSERT INTO `teacher` VALUES ('2','mathematics teacher');
 INSERT INTO `teacher` VALUES ('3','English teacher');
 INSERT INTO `teacher` VALUES ('4','Physical Education Teacher');
 INSERT INTO `teacher` VALUES ('5','Music Teacher');


CREATE TABLE `stu_teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `stu_id` int(11) DEFAULT NULL,
  `teacher_id` int(11) DEFAULT NULL,
     `flag` int(11) DEFAULT '0' COMMENT '0-valid 1-invalid',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=14 DEFAULT CHARSET=utf8;

INSERT INTO `stu_teacher` VALUES ('1', '1', '1', '0');
INSERT INTO `stu_teacher` VALUES ('2', '1', '2', '0');
INSERT INTO `stu_teacher` VALUES ('3', '1', '3', '0');
INSERT INTO `stu_teacher` VALUES ('4', '2', '2', '0');
INSERT INTO `stu_teacher` VALUES ('5', '2', '3', '0');
INSERT INTO `stu_teacher` VALUES ('6', '2', '4', '0');
INSERT INTO `stu_teacher` VALUES ('7', '2', '5', '0');
INSERT INTO `stu_teacher` VALUES ('8', '3', '1', '0');
INSERT INTO `stu_teacher` VALUES ('9', '3', '2', '0');
INSERT INTO `stu_teacher` VALUES ('10', '3', '3', '0');
INSERT INTO `stu_teacher` VALUES ('11', '3', '4', '0');
INSERT INTO `stu_teacher` VALUES ('12', '3', '5', '0');
INSERT INTO `stu_teacher` VALUES ('13', '4', '1', '1');

Now we need to find out which teachers each student has according to the group of students, one piece of data for each student, and each student can display it

At the beginning sql was

SELECT
	s.`name` stuName,
	GROUP_CONCAT(t.`name` SEPARATOR ',') as teacherName
FROM
	student s
LEFT JOIN stu_teacher st ON s.id = st.stu_id
LEFT JOIN teacher t ON st.teacher_id = t.id
GROUP BY
	s.id

The result is as follows

Later, the business requirements only take the valid data of the relational table, and each student must show

Modify sql to

SELECT
	s.`name` stuName,
	GROUP_CONCAT(t.`name` SEPARATOR ',') as teacherName
FROM
	student s
LEFT JOIN stu_teacher st ON s.id = st.stu_id
LEFT JOIN teacher t ON st.teacher_id = t.id
where st.flag = 0
GROUP BY
	s.id

The query result is:

As a result, Zhao Liu disappeared. . . . .

Later, I found that the GROUP_CONCAT method can use the case when function, so I modified the sql to

SELECT
	s.`name` stuName,
	GROUP_CONCAT(
		(
			CASE st.flag
			WHEN 0 THEN
				t.`name`
			ELSE
				NULL
			END
		) SEPARATOR ','
	) AS teacherName
FROM
	student s
LEFT JOIN stu_teacher st ON s.id = st.stu_id
LEFT JOIN teacher t ON st.teacher_id = t.id
GROUP BY
	s.id

The query results are as follows

Meet demand