COALESCE ( s.sname, '组合' ) AS sname (ISNULL 时默认值)

SELECT
c.id,
c.cname,
c.subject_id,
c.title,
c.front_cover,
c.start_time,
c.introduction,
c.deadline,
c.type,
c.STATUS,
c.teacher_name AS tName,
COALESCE ( s.sname, ‘组合’ ) AS sname
FROM
cmf_course_registration AS r
LEFT JOIN cmf_course AS c ON r.cid = c.course_id
LEFT JOIN cmf_subject AS s ON c.subject_id = s.id
WHERE
c.is_lu = 0
AND r.uid = 16100
AND r.STATUS = 1;