sql查询存在 01 课程但可能不存在 02 课程的情况不存在时显示为 null

SELECT s.student_id, s.student_name, CASE WHEN c1.course_name = '01' THEN 'Yes' ELSE 'No' END AS 'has_01_course', CASE WHEN c2.course_name = '02' THEN c2.score ELSE NULL END AS 'score_of_02_course' FROM students s LEFT JOIN scores c1 ON s.student_id = c1.student_id AND c1.course_name = '01' LEFT JOIN scores c2 ON s.student_id = c2.student_id AND c2.course_name = '02'