MySQL多表选择语句

Question

This is just a test project. I want to know how to select All professors with more than 5 failed students in a subject

I already know how to select All professors with at least 2 subjects with the following query:

SELECT    paulin_professors.*,
          IFNULL(sub_p.total, 0) num
FROM      paulin_professors
LEFT JOIN ( SELECT   COUNT(*) total, pau_profid
            FROM     paulin_profsubject
            GROUP BY pau_profid
          ) sub_p ON (sub_p.pau_profid = paulin_professors.pau_profid)
    WHERE sub_p.total >= 2;

I know I'm close but I can't get it to work (All professors with more than 5 failed students in a subject) . Any ideas? TIA

Answer 1

尝试将SELECT与UNION一起使用

select [columnName1],[columnName2] from [Table1] where [condition] union select [columnName1],[columnName2] from [Table1] where [condition] union ....

Answer 2

Looks like can get the professor IDs from the profsubject table and JOIN the studentenrolled table using the subjid for the join. In a similar way to what you had, you can get the count of students who have a grade less than a certain pass/fail threshold (in this case 65).

Then to get a short list, you can select the distinct profids from this derivaed table.

SELECT
    distinct pau_profid
FROM
    (SELECT    
        t1.pau_profid,
        IFNULL(t2.total_failed, 0) number_failed >= 5
    FROM      
        paulin_profsubject t1
    LEFT JOIN 
        (SELECT   
            COUNT(*) total_failed, 
            pau_subjid
        FROM     
            paulin_studentenrolled
        WHERE
            pau_grade < 65
        GROUP BY 
            pau_subjid
        ) t2
    ON 
        t1.pau_subjid = t2.pau_subjid
    WHERE 
        number_failed >= 5
    ) t3;