-- 4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩 -- (包括有成绩的和无成绩的)

SELECT
st.s_id,
st.s_name,
CASE WHEN AVG(sc.s_score) IS  NULL  THEN '无' ELSE ROUND (AVG(sc.s_score) ,2) END AS '平均分'
FROM  student st
LEFT JOIN score sc ON sc.s_id=st.s_id
GROUP BY st.s_id 
HAVING AVG(sc.s_score)<60  OR  AVG(sc.s_score) IS NULL