MySQL 的 Join 语句

Question

我有一个包含以下表格的 MySQL 数据库：

Table: Professor
Attributes: ID, ProfessorName

Table: Class
Attributes: ID, ClassName, SubjectID (secondary)

Table: Subject
Attributes: ID, SubjectName

Table: DocCategory
Attributes: ID, DocCategoryName

Table: Document
Attributes: ID, DocName, ProfessorID (secondary), ClassID (secondary), DocCategoryID (secondary)

有人可以通过查询为我指明正确的方向，以便我获得包含以下内容的报告：

所有 ProfessorName 的列表，第二列包含文档表中与该教授相关的文档计数？

提前致谢！

Answer 1

用途：

  SELECT p.professorname,
         COUNT(*) AS numDocuments
    FROM PROFESSOR p
    JOIN DOCUMENT d ON d.professorid = p.id
GROUP BY p.professorname

要查看所有教授的列表——没有文件的教授将列出零值——使用：

   SELECT p.professorname,
          COALESCE(COUNT(d.id), 0) AS numDocuments
     FROM PROFESSOR p
LEFT JOIN DOCUMENT d ON d.professorid = p.id
 GROUP BY p.professorname