Question

I have a MySQL database containing the following tables:

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)

Can someone point me in the right direction with a query so that I get a report containing:

a list of all ProfessorName with a second column containing a count for documents in the document table related to that professor?

Thanks in advance!

Answer 1

Use:

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

To see a list of all professors — those without documents will list a value of zero — use:

   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