sql - LEFT JOIN 混淆——我需要检索学生人数

Question

我有四个表:

students
classes
teachers
teacher_assignments

classes 和 teachers 具有多对多关系，因此 teacher_assignments 充当 xref 表(包含字段 teacher_id 和 class_id)。

students 中的每个学生都有一个 class_id(多对一 - 许多学生到一个类(class))。

我还应该提到 teacher_assignments 有一个 active 列 (BOOL)，它指示该作业当前是否处于事件状态

---

我想做什么:

我想检索以下内容:

• class_name -- 其级别和子级别的连接，例如3 和 A
• teacher_names -- 当前分配给该类(class)的教师姓名
• student_count -- 每个类(class)的学生人数

起初，我尝试只检索 class_name 和 teacher_names，如下所示:

SELECT
    CONCAT(CONVERT(classes.level, CHAR(8)), classes.sub_level) AS class_name,
    GROUP_CONCAT(DISTINCT teachers.name SEPARATOR ',') AS teacher_names

FROM
    teacher_assignments
        LEFT JOIN teachers
            ON teachers.id = teacher_assignments.teacher_id
            AND teacher_assignments.active = TRUE
        LEFT JOIN classes
            ON classes.id = teacher_assignments.class_id

GROUP BY classes.id

这工作正常并输出:

 class_name | teacher_names
 --------------------------------------
 1A         | NULL
 2A         | John, Sam
 3B         | Sam, Sarah

(类(class) 1A 目前没有老师，因此预计为 NULL)

... 但是，现在我不知道如何将 student_count 加入其中。

---

我的问题:

在上面的查询中，students 表究竟应该如何与其他表连接，以便生成 student_count 列？

Answer 1

使用:

   SELECT CONCAT(CONVERT(c.level, CHAR(8)), c.sub_level) AS class_name,
          GROUP_CONCAT(DISTINCT teachers.name SEPARATOR ',') AS teacher_names,
          COUNT(s.id) AS studentCount
     FROM CLASSES c
LEFT JOIN TEACHER_ASSIGNMENTS ta ON ta.class_id = c.id
                                AND ta.active = TRUE
LEFT JOIN TEACHERS t ON t.id = ta.teacher_id
LEFT JOIN STUDENTS s ON s.class_id = c.id
 GROUP BY class_name

使用 MySQL 时，可以在 GROUP BY 中引用列别名，否则您必须复制生成 class_name 列值的逻辑。这也是 GROUP 所在的列，因为 GROUP_CONCAT 和 COUNT 是聚合函数。

要获得零作为计数值，您可能需要使用:

   SELECT CONCAT(CONVERT(c.level, CHAR(8)), c.sub_level) AS class_name,
          GROUP_CONCAT(DISTINCT teachers.name SEPARATOR ',') AS teacher_names,
          COALESCE(COUNT(s.id), 0) AS studentCount
     FROM CLASSES c
LEFT JOIN TEACHER_ASSIGNMENTS ta ON ta.class_id = c.id
                                AND ta.active = TRUE
LEFT JOIN TEACHERS t ON t.id = ta.teacher_id
LEFT JOIN STUDENTS s ON s.class_id = c.id
 GROUP BY class_name