加入語句的MySQL

Question

我有一個包含以下各表MySQL數據庫：

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) 

有人點我在查詢正確的方向，使我得到含有報道：

列表所有教授姓名都帶有第二列，其中包含與該教授有關的文件表中的文件計數？

提前致謝！

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 

要看到所有的教授名單 - 那些沒有文件將列出的零值 - 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