无法在 SQL Server 中绑定多部分标识符并混淆

Question

I have this SQL query:

SELECT 
    stu.sno, sname, cname
FROM
    sc scc,
    (SELECT AVG(sc.grade) AS avg_grade 
     FROM sc 
     GROUP BY sc.cno) AS avg_grades
INNER JOIN 
    course c ON c.cno = scc.cno
INNER JOIN 
    s stu ON stu.sno = scc.sno;

And there is an error that the multi-part identifier scc.cno could not be bound. I'm confused - could someone help me?

Answer 1

Don't mix implicit and explicit joins! Matter of fact, don't use implicit joins: this is archaic syntax, that should not appear in new code.

The comma in the FROM clause should (probably) be a CROSS JOIN :

SELECT stu.sno, sname, cname
FROM sc scc
CROSS JOIN (SELECT AVG(sc.grade) AS avg_grade FROM sc GROUP BY sc.cno) AS avg_grades
INNER JOIN course c on c.cno = scc.cno
INNER JOIN s stu on stu.sno = scc.sno;

Note that, for this subquery to be useful, you would probably need to select column avg_grade . I would also recommend prefixing each column with the table it belongs to, to remove any possible ambiguity.

Finally: you (probably) can use window functions instead of a subquery:

SELECT stu.sno, sname, cname, scc.
FROM (SELECT *, AVG(grade) OVER() avg_grade FROM sc) scc
INNER JOIN course c on c.cno = scc.cno
INNER JOIN s stu on stu.sno = scc.sno;

Answer 2

Assuming a one-to-many join of students and courses and joined table of student courses (ie, sc ), consider a simplified aggregation on joined tables. Be sure to always qualify columns with alias if query contains more than on table:

SELECT 
    s.sno AS student_number
    , s.sname AS student_name
    , c.cname AS course_name
    , AVG(sc.grade) AS avg_grade
FROM
    sc
INNER JOIN 
    course c ON c.cno = sc.cno
INNER JOIN 
    stu s ON s.sno = sc.sno
GROUP BY
    s.sno
    , s.sname
    , c.cname