SELECT*FROMlessonllLEFTJOIN((SELECTbeforelessonnumFROMlessonWHEREbeforelessonnumisnot... SELECT *
FROM lesson ll LEFT JOIN
((SELECT beforelessonnum
FROM lesson
WHERE beforelessonnum is not null) a JOIN( SELECT lessonnum,lessonname,year,term,credit,teachernum FROM lesson) l on a.beforelessonnum = l.lessonnum) b ON ll.beforelessonnum =b.lessonnum;
报0RA-00905 报错位置在 l.lessonnum) b ON ll中的b那里。