我想知道一个班有多少学生。如果有记录，以上工作正常。
但是，如果没有结果，则返回一个“记录”，其中包含所有字段 NULL 除了 students_count 返回零

+---+------+------+----------------+----------------+
|   | id   | name | professor_name | students_count |
+---+------+------+----------------+----------------+
| 1 | null | null | null           | 0              |
+---+------+------+----------------+----------------+

我希望没有记录被退回。
如果一个记录返回，它看起来是这样的。

+---+----+-------------+----------------+----------------+
|   | id | name        | professor_name | students_count |
+---+----+-------------+----------------+----------------+
| 1 | 1  | Science 101 | Atkins         | 16             |
+---+----+-------------+----------------+----------------+

我已经试过了 LEFT JOIN 以及 IFNULL(COUNT(DISTINCT students.id), null) AS students_count 但我好像没法让它发挥作用。
有什么帮助吗？

SELECT  classes.*,
        professors.name AS professor_name,
        COUNT(DISTINCT students.id) AS students_count
FROM classes
INNER JOIN professors ON classes.professor_id = professors.id
LEFT JOIN students ON  classes.id = students.class_id AND classes.class_id IS NOT NULL
WHERE classes.class_id = 3

使用下面的泛型数据，class\u id为3，结果应该是no records。但是 null 返回计数为0的记录。

+---+------+------+----------------+----------------+
|   | id   | name | professor_name | students_count |
+---+------+------+----------------+----------------+
| 1 | null | null | null           | 0              |
+---+------+------+----------------+----------------+

使用类标识1将返回：

+---+----+-------------+----------------+----------------+
|   | id | name        | professor_name | students_count |
+---+----+-------------+----------------+----------------+
| 1 | 1  | Science 101 | Atkins         | 4              |
+---+----+-------------+----------------+----------------+

通用数据
班级

+---+-------------+---------------+
|   | name        | professors_id |
+---+-------------+---------------+
| 1 | Science 101 | 1             |
+---+-------------+---------------+
| 2 | Math        | 2             |
+---+-------------+---------------+
| 3 | English     | 3             |
+---+-------------+---------------+

教授

+----+--------+
| id | name   |
+----+--------+
| 1  | Atkins |
+----+--------+
| 2  | Button |
+----+--------+
| 3  | Castor |
+----+--------+

学生

+----+-------+------------+
| id | name  | classes_id |
+----+-------+------------+
| 1  | Adam  | 1          |
+----+-------+------------+
| 2  | Beth  | 1          |
+----+-------+------------+
| 3  | Chris | 1          |
+----+-------+------------+
| 4  | David | 1          |
+----+-------+------------+
| 5  | Erma  | 2          |
+----+-------+------------+