在有條件語句的多表上使用左連接

Question

我有一個MYSQL問題，它只能通過使用內部連接來解決，而我不是那麼擅長，我有四個表。

表1：教室 classroomID，姓名，.......

表2：教師 FNAME，MNAME，L-NAME，.......

表3：受檢 subjectID，名稱，classroom_id，.....

表4 Teachers_subjects teacher_id和subject_id

我希望有一個查詢來獲取我所有的類名（classrooms.name）及其所有科目名稱（subjects.name） 其中subjects.classroom_id = classrooms.classroomID

我也想獲取教師姓名（teachers.name）其中Teachers_subjects.subject_id = subject.subjectID 和Teachers_subjects.teacher_id = teachers.teacherID。

我目前使用此查詢：

SELECT classrooms.name, subjects.name, 
concat(teachers.fname,' ',teachers.mname,' ',teachers.lname) as Teachers_name 
FROM classrooms, subjects, teachers, teachers_subjects 
WHERE 
subjects.classroom_id=classrooms.classroomID 
AND 
teachers_subjects.teacher_id=teachers.teacherID 
AND 
teachers_subjects.subject_id=subjects.subjectID 

我得到我想要什麼，但我想即使它不是在桌子上teachers_subjects所有類及其相應的科目。這是列出所有班級及其科目，即使該科目沒有分配給任何教師。

感謝

Answer 1

你可以嘗試像

SELECT classrooms.name, 
    subjects.name, 
    concat(teachers.fname,' ',teachers.mname,' ',teachers.lname) as Teachers_name 
FROM classrooms INNER JOIN 
    subjects ON subjects.classroom_id=classrooms.classroomID LEFT JOIN 
    teachers_subjects ON teachers_subjects.subject_id=subjects.subjectID LEFT JOIN 
    teachers ON teachers_subjects.teacher_id=teachers.teacherID 

看一看這對於一個不錯的圖形引入的文章（Introduction to JOINs – Basic of JOINs）。