mySQL SELECT左外部联接涉及两个以上的表

Question

Given the following tables:

Students((PK)studentID, firstName, lastName, gender, dateOfBirth)

Courses((PK)courseCode, courseName, classLevel, credit)

Registration((PK,FK)studentID, (PK,FK)courseCode, grade)

I'd like to write a query that grabs the info for every student ( Students.studentID , Students.firstName , Students.lastName ), their course info ( Courses.courseName ), and grade ( Registration.grade ), including students who aren't registered for classes returning null values in the courseName and grade columns for those students.

From what I know (I'm still learning a lot) this will require something like:

Students LEFT OUTER JOIN [some aggregate of Courses and Registration]

But I'm not sure how to format it using mySQL syntax.

Here what I've got:

SELECT S.studentID, S.firstName, S.lastName, C.courseName, R.grade
    FROM Courses AS C, Students AS S
    LEFT OUTER JOIN Registration AS R ON S.studentID = R.studentID
    AND R.courseCode = C.courseCode
ORDER BY S.studentID
;

I know its not right I get a 'Unknown Column C.courseCode' error, so the syntax is obviously off, but this is as close as I could get to what I think it needs to look like.

Answer 1

Is this what you need? (I prefer LEFT JOIN because it is shorter than LEFT OUTER JOIN , but they are the same.)

SELECT S.studentID, S.firstName, S.lastName, C.courseName, R.grade
    FROM Students AS S
    LEFT JOIN Registration AS R ON S.studentID = R.studentID
    LEFT JOIN Courses AS C ON R.courseCode = C.courseCode
ORDER BY S.studentID
;

Edit: Like Gordon, I prefer explicit joins as you can see from my answer. However, to my knowledge there is no difference in performance.

Answer 2

When you use left join , start with table that contains all the rows you want to keep. In this case, students . Then use left join on all the other tables.

Also, never use commas in the from clause. Always use explicit join syntax.

So, your query should look like:

SELECT S.studentID, S.firstName, S.lastName, C.courseName, R.grade
FROM Students S LEFT OUTER JOIN
     Registration R
     ON S.studentID = R.studentID LEFT OUTER JOIN
     Courses AS C, 
     ON R.courseCode = C.courseCode
ORDER BY S.studentID;