MySQL如何将多个where子句添加到单个表

Question

我的桌子上有这个查询，它返回下面给出的表

SELECT sub.Subject_Name, count(sa.Attendance) AS Lectures_Attended FROM student_attendance sa
            INNER JOIN TeacherSubjects ts on (sa.TeacherSubject_ID = ts.TeacherSubject_ID)
            INNER JOIN Subjects sub on(ts.Subject_ID = sub.Subject_ID)
            where sa.Student_ID = 1 AND sub.Semester = '8th-Semester' AND sa.Attendance=1
            GROUP by sa.TeacherSubject_ID
UNION ALL

SELECT sub.Subject_Name, count(sa.Attendance) AS Total_Lectures FROM student_attendance sa
            INNER JOIN TeacherSubjects ts on (sa.TeacherSubject_ID = ts.TeacherSubject_ID)
            INNER JOIN Subjects sub on(ts.Subject_ID = sub.Subject_ID)
            where sa.Student_ID = 1 AND sub.Semester = '8th-Semester'
            GROUP by sa.TeacherSubject_ID

SubjectName LecturesAttended                                                     
    Php           2                                                             
    Php           3

现在我不需要第二行，即Php | 3

我需要这样显示表格

SubjectName  LecturesAttended  TotalLectures                              
   Php             2               3

Answer 1

您只需要将查询减少为：

SELECT 
    sub.Subject_Name, 
    COUNT(IF(sa.Attendance=1,1,NULL)) AS Lectures_Attended,
    COUNT(*) Total_Lectures
FROM student_attendance sa INNER JOIN TeacherSubjects ts ON (sa.TeacherSubject_ID = ts.TeacherSubject_ID)
INNER JOIN Subjects sub ON (ts.Subject_ID = sub.Subject_ID)
WHERE sa.Student_ID = 1 AND sub.Semester = '8th-Semester'
GROUP by sa.TeacherSubject_ID;

有关if函数在mysql中的工作方式，请参见MySQL IF() Function。