使用左连接 left join
SELECT 
    a.stuid,
    a.stuname,
    a.course,
    a.source,
  a.classname
FROM (
SELECT
    student.stuid,
    student.stuname,
    source.course,
    source.source,
  class.classname
FROM
    student
LEFT JOIN class ON class.classid = student.classid
LEFT JOIN source ON
student.stuid = source.stuid
WHERE class.classname = ‘一班‘ ) AS a
LEFT JOIN (
SELECT
    student.stuid,
    student.stuname,
    source.course,
    source.source,
  class.classname
FROM
    student
LEFT JOIN class ON class.classid = student.classid
LEFT JOIN source ON
student.stuid = source.stuid
WHERE class.classname = ‘一班‘ ) AS b 
ON a.source < b.source 
AND a.course = b.course 
WHERE
    b.course IS NULL